[v2] linux: Add fallback for clone failure on posix_spawn (BZ#29115)
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Commit Message
Even though it might characterizeis as a kernel bug (incomplete kernel
support for CLONE_VM used along with CLONE_VFORK), time namespace support
is already presented in a some kernel releases (since 2020).
Although I agree with Carlos that it results in unexpected surprise
behaviour, I also see that providing a reasonable fallback that does not
add possible issues like race conditions (as some syscall fallbacks in the
past) provides a easier transition and make the interfaca to work on
multiple scenarios.
The fix itself does adds some corner cases that need to be handled, but
I still think it is maintanable and a general improvement.
--
Linux clone with CLONE_VM may fail for some namespace restriction (for
instance if kernel does not allow processes in different time namespaces
to share the sameaddress space).
In this case clone fails with EINVAL and posix_spawn can not spawn a new
process. However the same process can be spawned with fork and exec.
The patch fixes by retrying the clone syscall with just CLONE_VFORK
if clone fails with a non transient failure (ENOMEM and EAGAIN still
returns failure to caller).
Error on preparation phase or execve is returned by a pipe now that
there is no shared memory that ca be used. It requires some extra care
for some file operations:
* If the file operation would clobber the pipe file descriptor, the
helper process dup the pipe onto an unoccupied file descriptor.
* dup2 file action that targets the pipe file descriptor returns
EBADF.
* If closefrom argument overlaps the pipe file descriptor, it is
splited in two calls: [lowdp, pipe - 1] and [pipe + 1, ~Ou].
Failure on prepare phase in helper process does not trigger the fork
and exec fallback.
Checked on x86_64-linux-gnu.
---
sysdeps/unix/sysv/linux/spawni.c | 231 +++++++++++++++++++++++++------
1 file changed, 185 insertions(+), 46 deletions(-)
Comments
On Mai 03 2022, Adhemerval Zanella via Libc-alpha wrote:
> The patch fixes by retrying the clone syscall with just CLONE_VFORK
> if clone fails with a non transient failure (ENOMEM and EAGAIN still
> returns failure to caller).
I there any point for CLONE_VFORK without CLONE_VM? Since the VM isn't
shared, there should be no harm for both processes to run concurrently.
On 2022-05-03 16:46, Adhemerval Zanella wrote:
> Even though it might characterizeis as a kernel bug (incomplete kernel
> support for CLONE_VM used along with CLONE_VFORK), time namespace
> support
> is already presented in a some kernel releases (since 2020).
>
> Although I agree with Carlos that it results in unexpected surprise
> behaviour, I also see that providing a reasonable fallback that does
> not
> add possible issues like race conditions (as some syscall fallbacks in
> the
> past) provides a easier transition and make the interfaca to work on
> multiple scenarios.
>
> The fix itself does adds some corner cases that need to be handled, but
> I still think it is maintanable and a general improvement.
>
> --
>
> Linux clone with CLONE_VM may fail for some namespace restriction (for
> instance if kernel does not allow processes in different time
> namespaces
> to share the sameaddress space).
>
> In this case clone fails with EINVAL and posix_spawn can not spawn a
> new
> process. However the same process can be spawned with fork and exec.
>
> The patch fixes by retrying the clone syscall with just CLONE_VFORK
> if clone fails with a non transient failure (ENOMEM and EAGAIN still
> returns failure to caller).
>
> Error on preparation phase or execve is returned by a pipe now that
> there is no shared memory that ca be used. It requires some extra care
> for some file operations:
>
> * If the file operation would clobber the pipe file descriptor, the
> helper process dup the pipe onto an unoccupied file descriptor.
>
> * dup2 file action that targets the pipe file descriptor returns
> EBADF.
>
> * If closefrom argument overlaps the pipe file descriptor, it is
> splited in two calls: [lowdp, pipe - 1] and [pipe + 1, ~Ou].
>
> Failure on prepare phase in helper process does not trigger the fork
> and exec fallback.
>
> Checked on x86_64-linux-gnu.
> ---
> sysdeps/unix/sysv/linux/spawni.c | 231 +++++++++++++++++++++++++------
> 1 file changed, 185 insertions(+), 46 deletions(-)
>
> diff --git a/sysdeps/unix/sysv/linux/spawni.c
> b/sysdeps/unix/sysv/linux/spawni.c
> index d6f5ca89cd..e17b1967ef 100644
> --- a/sysdeps/unix/sysv/linux/spawni.c
> +++ b/sysdeps/unix/sysv/linux/spawni.c
> @@ -44,7 +44,11 @@
> third issue is done by a stack allocation in parent, and by using a
> field in struct spawn_args where the child can write an error
> code. CLONE_VFORK ensures that the parent does not run until the
> - child has either exec'ed successfully or exited. */
> + child has either exec'ed successfully or exited.
> +
> + If the clone with CLONE_VM and CLONE_VFORK fails (due any kernel
> limitation
> + such as time namespace), only CLONE_VFORK is used instead and the
> + preparation and execve failures are communicated with a pipe. */
>
>
> /* The Unix standard contains a long explanation of the way to signal
> @@ -67,6 +71,7 @@ struct posix_spawn_args
> char *const *envp;
> int xflags;
> int err;
> + int pipe[2];
> };
>
> /* Older version requires that shell script without shebang definition
> @@ -94,15 +99,59 @@ maybe_script_execute (struct posix_spawn_args
> *args)
> }
> }
>
> +/* If the file operation would clobber the pipe fd used to communite
> with
> + parent, dup the pipe onto an unoccupied file descriptor. */
> +static inline bool
> +spawni_fa_handle_pipe (const struct __spawn_action *fa, int p[])
> +{
> + int fd;
> +
> + switch (fa->tag)
> + {
> + case spawn_do_close:
> + fd = fa->action.close_action.fd;
> + break;
> + case spawn_do_open:
> + fd = fa->action.open_action.fd;
> + break;
> + case spawn_do_dup2:
> + fd = fa->action.open_action.fd;
"dup2_action.newfd" should be used here.
> + break;
> + case spawn_do_fchdir:
> + fd = fa->action.fchdir_action.fd;
> + default:
> + return true;
> + }
> +
> + if (fd == p[1])
> + {
> + int r = __fcntl (p[1], F_DUPFD_CLOEXEC);
> + if (r < 0)
> + return false;
> + __close_nocancel (p[1]);
> + p[1] = r;
> + }
> +
> + return true;
> +}
> +
> +static inline bool
> +spawni_fa_closerange (int from, int to)
> +{
> + int r = INLINE_SYSCALL_CALL (close_range, from, to, 0);
> + return r == 0 || __closefrom_fallback (from, false);
"__closefrom_fallback" ignores "to", so it will close the error pipe on
Linux kernels not supporting close_range().
> +}
> +
> /* Function used in the clone call to setup the signals mask,
> posix_spawn
> attributes, and file actions. It run on its own stack (provided by
> the
> posix_spawn call). */
> -static int
> -__spawni_child (void *arguments)
> +static _Noreturn int
> +spawni_child (void *arguments)
> {
> struct posix_spawn_args *args = arguments;
> const posix_spawnattr_t *restrict attr = args->attr;
> const posix_spawn_file_actions_t *file_actions = args->fa;
> + bool use_pipe = args->pipe[0] != -1;
>
> /* The child must ensure that no signal handler are enabled because
> it shared
> memory with parent, so the signal disposition must be either
> SIG_DFL or
> @@ -113,6 +162,9 @@ __spawni_child (void *arguments)
> struct sigaction sa;
> memset (&sa, '\0', sizeof (sa));
>
> + if (use_pipe)
> + __close (args->pipe[0]);
> +
> sigset_t hset;
> __sigprocmask (SIG_BLOCK, 0, &hset);
> for (int sig = 1; sig < _NSIG; ++sig)
> @@ -181,6 +233,9 @@ __spawni_child (void *arguments)
> {
> struct __spawn_action *action = &file_actions->__actions[cnt];
>
> + if (use_pipe && !spawni_fa_handle_pipe (action, args->pipe))
> + goto fail;
> +
> switch (action->tag)
> {
> case spawn_do_close:
> @@ -233,6 +288,11 @@ __spawni_child (void *arguments)
> break;
>
> case spawn_do_dup2:
> + if (use_pipe && action->action.dup2_action.fd == args->pipe[1])
> + {
> + errno = EBADF;
> + goto fail;
> + }
> /* Austin Group issue #411 requires adddup2 action with source
> and destination being equal to remove close-on-exec flag. */
> if (action->action.dup2_action.fd
> @@ -264,8 +324,16 @@ __spawni_child (void *arguments)
> case spawn_do_closefrom:
> {
> int lowfd = action->action.closefrom_action.from;
> - int r = INLINE_SYSCALL_CALL (close_range, lowfd, ~0U, 0);
> - if (r != 0 && !__closefrom_fallback (lowfd, false))
> + /* Skip the pipe descriptor if it is used. No need to handle
> + it since it created and duplicated with O_CLOEXEC. */
> + if (use_pipe
> + && args->pipe[1] > action->action.closefrom_action.from)
Maybe it's better to use "lowfd" instead of
"action->action.closefrom_action.from"?
> + {
> + if (!spawni_fa_closerange (lowfd, args->pipe[1] - 1)
> + || !spawni_fa_closerange (args->pipe[1] + 1, ~0U))
> + goto fail;
> + }
> + else if (!spawni_fa_closerange (lowfd, ~0U))
This will close the error pipe if "use_pipe && args->pipe[1] == low_fd".
> goto fail;
> } break;
>
> @@ -300,10 +368,114 @@ fail:
> (EINTERNALBUG) describing that, use ECHILD. Another option would
> be to set args->err to some negative sentinel and have the parent
> abort(), but that seems needlessly harsh. */
> - args->err = errno ? : ECHILD;
> + int ret = errno ? : ECHILD;
> + if (use_pipe)
> + {
> + while (__write_nocancel (args->pipe[1], &ret, sizeof (ret)) < 0)
> + if (errno == EPIPE || errno == EBADF)
> + break;
> + }
> + else
> + args->err = ret;
> +
> _exit (SPAWN_ERROR);
> }
>
> +static pid_t
> +clone_call (struct posix_spawn_args *args, int flags, void *stack,
> + size_t stack_size)
> +{
> + struct clone_args clone_args =
> + {
> + .flags = flags,
> + .exit_signal = SIGCHLD,
> + .stack = (uintptr_t) stack,
> + .stack_size = stack_size,
> + };
> + return __clone_internal (&clone_args, spawni_child, args);
> +}
> +
> +/* Spawn a new process using clone with CLONE_VM | CLONE_VFORK (to
> optimize
> + memory and overcommit) and return TRUE if the helper was created or
> if the
> + failure was not due resource exhaustion. */
> +static bool
> +spawni_clone (struct posix_spawn_args *args, pid_t *new_pid, int *ec,
> + void *stack, size_t stack_size)
> +{
> + /* The clone flags used will create a new child that will run in the
> same
> + memory space (CLONE_VM) and the execution of calling thread will
> be
> + suspend until the child calls execve or _exit.
> +
> + Also since the calling thread execution will be suspend, there is
> not
> + need for CLONE_SETTLS. Although parent and child share the same
> TLS
> + namespace, there will be no concurrent access for TLS variables
> (errno
> + for instance). */
> + *new_pid = clone_call (args, CLONE_VM | CLONE_VFORK, stack,
> stack_size);
> +
> + /* It needs to collect the case where the auxiliary process was
> created
> + but failed to execute the file (due either any preparation step
> or
> + for execve itself). */
> + if (*new_pid > 0)
> + {
> + /* Also, it handles the unlikely case where the auxiliary
> process was
> + terminated before calling execve as if it was successfully. The
> + args.err is set to 0 as default and changed to a positive value
> + only in case of failure, so in case of premature termination
> + due a signal args.err will remain zeroed and it will be up to
> + caller to actually collect it. */
> + *ec = args->err;
> + if (*ec > 0)
> + /* There still an unlikely case where the child is cancelled after
> + setting args.err, due to a positive error value. Also there is
> + possible pid reuse race (where the kernel allocated the same pid
> + to an unrelated process). Unfortunately due synchronization
> + issues where the kernel might not have the process collected
> + the waitpid below can not use WNOHANG. */
> + __waitpid (*new_pid, NULL, 0);
> + }
> + else
> + *ec = errno;
> +
> + /* There is no much point in retrying with fork and exec if kernel
> returns a
> + failure due resource exhaustion. */
> + return *new_pid > 0 || (errno == ENOMEM || errno == EAGAIN);
> +}
> +
> +/* Fallback spawn case which does not use CLONE_VM. Any prepration
> step or
> + execve failure is passed with a pipe, which requires additional
> care by
> + the helper stating process since it additional file descriptors
> handle. */
> +static void
> +spawni_vfork (struct posix_spawn_args *args, pid_t *new_pid, int *ec,
> + char *stack, size_t stack_size)
> +{
> + if (__pipe2 (args->pipe, O_CLOEXEC) != 0)
> + {
> + *ec = errno;
> + return;
> + }
> +
> + /* Do not trigger atfork handler nor any internal state reset since
> the
> + helper process will call execve. */
> + *new_pid = clone_call (args, CLONE_VFORK, stack, stack_size);
> + if (*new_pid == 0)
> + spawni_child (args);
spawni_child() is run in the child by clone_call().
> +
> + __close (args->pipe[1]);
> +
> + if (*new_pid > 0)
> + {
> + if (__read (args->pipe[0], ec, sizeof *ec) != sizeof *ec)
> + /* A successful execve will close the helper process pipe end. */
> + *ec = 0;
> + else
> + __waitpid (*new_pid, NULL, 0);
> + }
> + else
> + *ec = errno;
> +
> + __close (args->pipe[0]);
> +}
> +
> /* Spawn a new process executing PATH with the attributes describes in
> *ATTRP.
> Before running the process perform the actions described in
> FILE-ACTIONS. */
> static int
> @@ -367,49 +539,16 @@ __spawnix (pid_t * pid, const char *file,
> args.argc = argc;
> args.envp = envp;
> args.xflags = xflags;
> + args.pipe[0] = args.pipe[1] = -1;
>
> __libc_signal_block_all (&args.oldmask);
>
> - /* The clone flags used will create a new child that will run in the
> same
> - memory space (CLONE_VM) and the execution of calling thread will
> be
> - suspend until the child calls execve or _exit.
> -
> - Also since the calling thread execution will be suspend, there is
> not
> - need for CLONE_SETTLS. Although parent and child share the same
> TLS
> - namespace, there will be no concurrent access for TLS variables
> (errno
> - for instance). */
> - struct clone_args clone_args =
> - {
> - .flags = CLONE_VM | CLONE_VFORK,
> - .exit_signal = SIGCHLD,
> - .stack = (uintptr_t) stack,
> - .stack_size = stack_size,
> - };
> - new_pid = __clone_internal (&clone_args, __spawni_child, &args);
> -
> - /* It needs to collect the case where the auxiliary process was
> created
> - but failed to execute the file (due either any preparation step
> or
> - for execve itself). */
> - if (new_pid > 0)
> - {
> - /* Also, it handles the unlikely case where the auxiliary
> process was
> - terminated before calling execve as if it was successfully. The
> - args.err is set to 0 as default and changed to a positive value
> - only in case of failure, so in case of premature termination
> - due a signal args.err will remain zeroed and it will be up to
> - caller to actually collect it. */
> - ec = args.err;
> - if (ec > 0)
> - /* There still an unlikely case where the child is cancelled after
> - setting args.err, due to a positive error value. Also there is
> - possible pid reuse race (where the kernel allocated the same pid
> - to an unrelated process). Unfortunately due synchronization
> - issues where the kernel might not have the process collected
> - the waitpid below can not use WNOHANG. */
> - __waitpid (new_pid, NULL, 0);
> - }
> - else
> - ec = errno;
> + /* clone with CLONE_VM | CLONE_VFORK may fail for some namespace
> restriction
> + (for instance Linux does not allow processes in different time
> namespaces
> + to share address space) and in this case clone fails with EINVAL.
> Retry
> + with fork and exec. */
> + if (!spawni_clone (&args, &new_pid, &ec, stack, stack_size))
> + spawni_vfork (&args, &new_pid, &ec, stack, stack_size);
"spawni_vfork" seems like a very confusing name (especially in
conjunction with the comment above), since here it means "use
CLONE_VFORK without CLONE_VM" (which is more like fork()) instead of
"use vfork()".
>
> __munmap (stack, stack_size);
I'm fine with the general approach (though the previous approach with
_Fork()/exec() was also fine :) )
There are also some typos in comments, but anyway, in my understanding,
there is currently no agreement among glibc developers on whether the
issue should be fixed on the glibc side. There is a discussion at [1]
which will hopefully attract kernel developers for clarifications. In
particular, I'm very interested whether the kernel can commit to not
breaking the ability to use vfork() instead of fork() any further.
Alexey
[1] https://www.openwall.com/lists/musl/2022/05/02/1
On 2022-05-03 17:16, Andreas Schwab wrote:
> On Mai 03 2022, Adhemerval Zanella via Libc-alpha wrote:
>
>> The patch fixes by retrying the clone syscall with just CLONE_VFORK
>> if clone fails with a non transient failure (ENOMEM and EAGAIN still
>> returns failure to caller).
>
> I there any point for CLONE_VFORK without CLONE_VM? Since the VM isn't
> shared, there should be no harm for both processes to run concurrently.
Not the patch author, but in case of posix_spawn() processes won't run
concurrently in a meaningful way because the parent will block on read()
from the error pipe. So it seems like a moot point.
Alexey
On 03/05/2022 13:06, Alexey Izbyshev wrote:
> On 2022-05-03 16:46, Adhemerval Zanella wrote:
>> Even though it might characterizeis as a kernel bug (incomplete kernel
>> support for CLONE_VM used along with CLONE_VFORK), time namespace support
>> is already presented in a some kernel releases (since 2020).
>>
>> Although I agree with Carlos that it results in unexpected surprise
>> behaviour, I also see that providing a reasonable fallback that does not
>> add possible issues like race conditions (as some syscall fallbacks in the
>> past) provides a easier transition and make the interfaca to work on
>> multiple scenarios.
>>
>> The fix itself does adds some corner cases that need to be handled, but
>> I still think it is maintanable and a general improvement.
>>
>> --
>>
>> Linux clone with CLONE_VM may fail for some namespace restriction (for
>> instance if kernel does not allow processes in different time namespaces
>> to share the sameaddress space).
>>
>> In this case clone fails with EINVAL and posix_spawn can not spawn a new
>> process. However the same process can be spawned with fork and exec.
>>
>> The patch fixes by retrying the clone syscall with just CLONE_VFORK
>> if clone fails with a non transient failure (ENOMEM and EAGAIN still
>> returns failure to caller).
>>
>> Error on preparation phase or execve is returned by a pipe now that
>> there is no shared memory that ca be used. It requires some extra care
>> for some file operations:
>>
>> * If the file operation would clobber the pipe file descriptor, the
>> helper process dup the pipe onto an unoccupied file descriptor.
>>
>> * dup2 file action that targets the pipe file descriptor returns
>> EBADF.
>>
>> * If closefrom argument overlaps the pipe file descriptor, it is
>> splited in two calls: [lowdp, pipe - 1] and [pipe + 1, ~Ou].
>>
>> Failure on prepare phase in helper process does not trigger the fork
>> and exec fallback.
>>
>> Checked on x86_64-linux-gnu.
>> ---
>> sysdeps/unix/sysv/linux/spawni.c | 231 +++++++++++++++++++++++++------
>> 1 file changed, 185 insertions(+), 46 deletions(-)
>>
>> diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c
>> index d6f5ca89cd..e17b1967ef 100644
>> --- a/sysdeps/unix/sysv/linux/spawni.c
>> +++ b/sysdeps/unix/sysv/linux/spawni.c
>> @@ -44,7 +44,11 @@
>> third issue is done by a stack allocation in parent, and by using a
>> field in struct spawn_args where the child can write an error
>> code. CLONE_VFORK ensures that the parent does not run until the
>> - child has either exec'ed successfully or exited. */
>> + child has either exec'ed successfully or exited.
>> +
>> + If the clone with CLONE_VM and CLONE_VFORK fails (due any kernel limitation
>> + such as time namespace), only CLONE_VFORK is used instead and the
>> + preparation and execve failures are communicated with a pipe. */
>>
>>
>> /* The Unix standard contains a long explanation of the way to signal
>> @@ -67,6 +71,7 @@ struct posix_spawn_args
>> char *const *envp;
>> int xflags;
>> int err;
>> + int pipe[2];
>> };
>>
>> /* Older version requires that shell script without shebang definition
>> @@ -94,15 +99,59 @@ maybe_script_execute (struct posix_spawn_args *args)
>> }
>> }
>>
>> +/* If the file operation would clobber the pipe fd used to communite with
>> + parent, dup the pipe onto an unoccupied file descriptor. */
>> +static inline bool
>> +spawni_fa_handle_pipe (const struct __spawn_action *fa, int p[])
>> +{
>> + int fd;
>> +
>> + switch (fa->tag)
>> + {
>> + case spawn_do_close:
>> + fd = fa->action.close_action.fd;
>> + break;
>> + case spawn_do_open:
>> + fd = fa->action.open_action.fd;
>> + break;
>> + case spawn_do_dup2:
>> + fd = fa->action.open_action.fd;
>
> "dup2_action.newfd" should be used here.
Ack.
>
>> + break;
>> + case spawn_do_fchdir:
>> + fd = fa->action.fchdir_action.fd;
>> + default:
>> + return true;
>> + }
>> +
>> + if (fd == p[1])
>> + {
>> + int r = __fcntl (p[1], F_DUPFD_CLOEXEC);
>> + if (r < 0)
>> + return false;
>> + __close_nocancel (p[1]);
>> + p[1] = r;
>> + }
>> +
>> + return true;
>> +}
>> +
>> +static inline bool
>> +spawni_fa_closerange (int from, int to)
>> +{
>> + int r = INLINE_SYSCALL_CALL (close_range, from, to, 0);
>> + return r == 0 || __closefrom_fallback (from, false);
>
> "__closefrom_fallback" ignores "to", so it will close the error pipe on Linux kernels not supporting close_range().
Sigh, I forgot about it.
>
>> +}
>> +
>> /* Function used in the clone call to setup the signals mask, posix_spawn
>> attributes, and file actions. It run on its own stack (provided by the
>> posix_spawn call). */
>> -static int
>> -__spawni_child (void *arguments)
>> +static _Noreturn int
>> +spawni_child (void *arguments)
>> {
>> struct posix_spawn_args *args = arguments;
>> const posix_spawnattr_t *restrict attr = args->attr;
>> const posix_spawn_file_actions_t *file_actions = args->fa;
>> + bool use_pipe = args->pipe[0] != -1;
>>
>> /* The child must ensure that no signal handler are enabled because it shared
>> memory with parent, so the signal disposition must be either SIG_DFL or
>> @@ -113,6 +162,9 @@ __spawni_child (void *arguments)
>> struct sigaction sa;
>> memset (&sa, '\0', sizeof (sa));
>>
>> + if (use_pipe)
>> + __close (args->pipe[0]);
>> +
>> sigset_t hset;
>> __sigprocmask (SIG_BLOCK, 0, &hset);
>> for (int sig = 1; sig < _NSIG; ++sig)
>> @@ -181,6 +233,9 @@ __spawni_child (void *arguments)
>> {
>> struct __spawn_action *action = &file_actions->__actions[cnt];
>>
>> + if (use_pipe && !spawni_fa_handle_pipe (action, args->pipe))
>> + goto fail;
>> +
>> switch (action->tag)
>> {
>> case spawn_do_close:
>> @@ -233,6 +288,11 @@ __spawni_child (void *arguments)
>> break;
>>
>> case spawn_do_dup2:
>> + if (use_pipe && action->action.dup2_action.fd == args->pipe[1])
>> + {
>> + errno = EBADF;
>> + goto fail;
>> + }
>> /* Austin Group issue #411 requires adddup2 action with source
>> and destination being equal to remove close-on-exec flag. */
>> if (action->action.dup2_action.fd
>> @@ -264,8 +324,16 @@ __spawni_child (void *arguments)
>> case spawn_do_closefrom:
>> {
>> int lowfd = action->action.closefrom_action.from;
>> - int r = INLINE_SYSCALL_CALL (close_range, lowfd, ~0U, 0);
>> - if (r != 0 && !__closefrom_fallback (lowfd, false))
>> + /* Skip the pipe descriptor if it is used. No need to handle
>> + it since it created and duplicated with O_CLOEXEC. */
>> + if (use_pipe
>> + && args->pipe[1] > action->action.closefrom_action.from)
>
> Maybe it's better to use "lowfd" instead of "action->action.closefrom_action.from"?
Ack.
>
>> + {
>> + if (!spawni_fa_closerange (lowfd, args->pipe[1] - 1)
>> + || !spawni_fa_closerange (args->pipe[1] + 1, ~0U))
>> + goto fail;
>> + }
>> + else if (!spawni_fa_closerange (lowfd, ~0U))
>
> This will close the error pipe if "use_pipe && args->pipe[1] == low_fd".
Ack.
>
>> goto fail;
>> } break;
>>
>> @@ -300,10 +368,114 @@ fail:
>> (EINTERNALBUG) describing that, use ECHILD. Another option would
>> be to set args->err to some negative sentinel and have the parent
>> abort(), but that seems needlessly harsh. */
>> - args->err = errno ? : ECHILD;
>> + int ret = errno ? : ECHILD;
>> + if (use_pipe)
>> + {
>> + while (__write_nocancel (args->pipe[1], &ret, sizeof (ret)) < 0)
>> + if (errno == EPIPE || errno == EBADF)
>> + break;
>> + }
>> + else
>> + args->err = ret;
>> +
>> _exit (SPAWN_ERROR);
>> }
>>
>> +static pid_t
>> +clone_call (struct posix_spawn_args *args, int flags, void *stack,
>> + size_t stack_size)
>> +{
>> + struct clone_args clone_args =
>> + {
>> + .flags = flags,
>> + .exit_signal = SIGCHLD,
>> + .stack = (uintptr_t) stack,
>> + .stack_size = stack_size,
>> + };
>> + return __clone_internal (&clone_args, spawni_child, args);
>> +}
>> +
>> +/* Spawn a new process using clone with CLONE_VM | CLONE_VFORK (to optimize
>> + memory and overcommit) and return TRUE if the helper was created or if the
>> + failure was not due resource exhaustion. */
>> +static bool
>> +spawni_clone (struct posix_spawn_args *args, pid_t *new_pid, int *ec,
>> + void *stack, size_t stack_size)
>> +{
>> + /* The clone flags used will create a new child that will run in the same
>> + memory space (CLONE_VM) and the execution of calling thread will be
>> + suspend until the child calls execve or _exit.
>> +
>> + Also since the calling thread execution will be suspend, there is not
>> + need for CLONE_SETTLS. Although parent and child share the same TLS
>> + namespace, there will be no concurrent access for TLS variables (errno
>> + for instance). */
>> + *new_pid = clone_call (args, CLONE_VM | CLONE_VFORK, stack, stack_size);
>> +
>> + /* It needs to collect the case where the auxiliary process was created
>> + but failed to execute the file (due either any preparation step or
>> + for execve itself). */
>> + if (*new_pid > 0)
>> + {
>> + /* Also, it handles the unlikely case where the auxiliary process was
>> + terminated before calling execve as if it was successfully. The
>> + args.err is set to 0 as default and changed to a positive value
>> + only in case of failure, so in case of premature termination
>> + due a signal args.err will remain zeroed and it will be up to
>> + caller to actually collect it. */
>> + *ec = args->err;
>> + if (*ec > 0)
>> + /* There still an unlikely case where the child is cancelled after
>> + setting args.err, due to a positive error value. Also there is
>> + possible pid reuse race (where the kernel allocated the same pid
>> + to an unrelated process). Unfortunately due synchronization
>> + issues where the kernel might not have the process collected
>> + the waitpid below can not use WNOHANG. */
>> + __waitpid (*new_pid, NULL, 0);
>> + }
>> + else
>> + *ec = errno;
>> +
>> + /* There is no much point in retrying with fork and exec if kernel returns a
>> + failure due resource exhaustion. */
>> + return *new_pid > 0 || (errno == ENOMEM || errno == EAGAIN);
>> +}
>> +
>> +/* Fallback spawn case which does not use CLONE_VM. Any prepration step or
>> + execve failure is passed with a pipe, which requires additional care by
>> + the helper stating process since it additional file descriptors handle. */
>> +static void
>> +spawni_vfork (struct posix_spawn_args *args, pid_t *new_pid, int *ec,
>> + char *stack, size_t stack_size)
>> +{
>> + if (__pipe2 (args->pipe, O_CLOEXEC) != 0)
>> + {
>> + *ec = errno;
>> + return;
>> + }
>> +
>> + /* Do not trigger atfork handler nor any internal state reset since the
>> + helper process will call execve. */
>> + *new_pid = clone_call (args, CLONE_VFORK, stack, stack_size);
>> + if (*new_pid == 0)
>> + spawni_child (args);
>
> spawni_child() is run in the child by clone_call().
Oops forgot to remove it.
>
>> +
>> + __close (args->pipe[1]);
>> +
>> + if (*new_pid > 0)
>> + {
>> + if (__read (args->pipe[0], ec, sizeof *ec) != sizeof *ec)
>> + /* A successful execve will close the helper process pipe end. */
>> + *ec = 0;
>> + else
>> + __waitpid (*new_pid, NULL, 0);
>> + }
>> + else
>> + *ec = errno;
>> +
>> + __close (args->pipe[0]);
>> +}
>> +
>> /* Spawn a new process executing PATH with the attributes describes in *ATTRP.
>> Before running the process perform the actions described in FILE-ACTIONS. */
>> static int
>> @@ -367,49 +539,16 @@ __spawnix (pid_t * pid, const char *file,
>> args.argc = argc;
>> args.envp = envp;
>> args.xflags = xflags;
>> + args.pipe[0] = args.pipe[1] = -1;
>>
>> __libc_signal_block_all (&args.oldmask);
>>
>> - /* The clone flags used will create a new child that will run in the same
>> - memory space (CLONE_VM) and the execution of calling thread will be
>> - suspend until the child calls execve or _exit.
>> -
>> - Also since the calling thread execution will be suspend, there is not
>> - need for CLONE_SETTLS. Although parent and child share the same TLS
>> - namespace, there will be no concurrent access for TLS variables (errno
>> - for instance). */
>> - struct clone_args clone_args =
>> - {
>> - .flags = CLONE_VM | CLONE_VFORK,
>> - .exit_signal = SIGCHLD,
>> - .stack = (uintptr_t) stack,
>> - .stack_size = stack_size,
>> - };
>> - new_pid = __clone_internal (&clone_args, __spawni_child, &args);
>> -
>> - /* It needs to collect the case where the auxiliary process was created
>> - but failed to execute the file (due either any preparation step or
>> - for execve itself). */
>> - if (new_pid > 0)
>> - {
>> - /* Also, it handles the unlikely case where the auxiliary process was
>> - terminated before calling execve as if it was successfully. The
>> - args.err is set to 0 as default and changed to a positive value
>> - only in case of failure, so in case of premature termination
>> - due a signal args.err will remain zeroed and it will be up to
>> - caller to actually collect it. */
>> - ec = args.err;
>> - if (ec > 0)
>> - /* There still an unlikely case where the child is cancelled after
>> - setting args.err, due to a positive error value. Also there is
>> - possible pid reuse race (where the kernel allocated the same pid
>> - to an unrelated process). Unfortunately due synchronization
>> - issues where the kernel might not have the process collected
>> - the waitpid below can not use WNOHANG. */
>> - __waitpid (new_pid, NULL, 0);
>> - }
>> - else
>> - ec = errno;
>> + /* clone with CLONE_VM | CLONE_VFORK may fail for some namespace restriction
>> + (for instance Linux does not allow processes in different time namespaces
>> + to share address space) and in this case clone fails with EINVAL. Retry
>> + with fork and exec. */
>> + if (!spawni_clone (&args, &new_pid, &ec, stack, stack_size))
>> + spawni_vfork (&args, &new_pid, &ec, stack, stack_size);
>
> "spawni_vfork" seems like a very confusing name (especially in conjunction with the comment above), since here it means "use CLONE_VFORK without CLONE_VM" (which is more like fork()) instead of "use vfork()".
I will rename again to spawni_fork, it makes more sense.
>>
>> __munmap (stack, stack_size);
>
> I'm fine with the general approach (though the previous approach with _Fork()/exec() was also fine :) )
>
> There are also some typos in comments, but anyway, in my understanding, there is currently no agreement among glibc developers on whether the issue should be fixed on the glibc side. There is a discussion at [1] which will hopefully attract kernel developers for clarifications. In particular, I'm very interested whether the kernel can commit to not breaking the ability to use vfork() instead of fork() any further.
I think to have a working solution if kernel decides to not support it
it a good thing to have, even if we decide to not fix it on glibc.
>
> Alexey
>
> [1] https://www.openwall.com/lists/musl/2022/05/02/1
@@ -44,7 +44,11 @@
third issue is done by a stack allocation in parent, and by using a
field in struct spawn_args where the child can write an error
code. CLONE_VFORK ensures that the parent does not run until the
- child has either exec'ed successfully or exited. */
+ child has either exec'ed successfully or exited.
+
+ If the clone with CLONE_VM and CLONE_VFORK fails (due any kernel limitation
+ such as time namespace), only CLONE_VFORK is used instead and the
+ preparation and execve failures are communicated with a pipe. */
/* The Unix standard contains a long explanation of the way to signal
@@ -67,6 +71,7 @@ struct posix_spawn_args
char *const *envp;
int xflags;
int err;
+ int pipe[2];
};
/* Older version requires that shell script without shebang definition
@@ -94,15 +99,59 @@ maybe_script_execute (struct posix_spawn_args *args)
}
}
+/* If the file operation would clobber the pipe fd used to communite with
+ parent, dup the pipe onto an unoccupied file descriptor. */
+static inline bool
+spawni_fa_handle_pipe (const struct __spawn_action *fa, int p[])
+{
+ int fd;
+
+ switch (fa->tag)
+ {
+ case spawn_do_close:
+ fd = fa->action.close_action.fd;
+ break;
+ case spawn_do_open:
+ fd = fa->action.open_action.fd;
+ break;
+ case spawn_do_dup2:
+ fd = fa->action.open_action.fd;
+ break;
+ case spawn_do_fchdir:
+ fd = fa->action.fchdir_action.fd;
+ default:
+ return true;
+ }
+
+ if (fd == p[1])
+ {
+ int r = __fcntl (p[1], F_DUPFD_CLOEXEC);
+ if (r < 0)
+ return false;
+ __close_nocancel (p[1]);
+ p[1] = r;
+ }
+
+ return true;
+}
+
+static inline bool
+spawni_fa_closerange (int from, int to)
+{
+ int r = INLINE_SYSCALL_CALL (close_range, from, to, 0);
+ return r == 0 || __closefrom_fallback (from, false);
+}
+
/* Function used in the clone call to setup the signals mask, posix_spawn
attributes, and file actions. It run on its own stack (provided by the
posix_spawn call). */
-static int
-__spawni_child (void *arguments)
+static _Noreturn int
+spawni_child (void *arguments)
{
struct posix_spawn_args *args = arguments;
const posix_spawnattr_t *restrict attr = args->attr;
const posix_spawn_file_actions_t *file_actions = args->fa;
+ bool use_pipe = args->pipe[0] != -1;
/* The child must ensure that no signal handler are enabled because it shared
memory with parent, so the signal disposition must be either SIG_DFL or
@@ -113,6 +162,9 @@ __spawni_child (void *arguments)
struct sigaction sa;
memset (&sa, '\0', sizeof (sa));
+ if (use_pipe)
+ __close (args->pipe[0]);
+
sigset_t hset;
__sigprocmask (SIG_BLOCK, 0, &hset);
for (int sig = 1; sig < _NSIG; ++sig)
@@ -181,6 +233,9 @@ __spawni_child (void *arguments)
{
struct __spawn_action *action = &file_actions->__actions[cnt];
+ if (use_pipe && !spawni_fa_handle_pipe (action, args->pipe))
+ goto fail;
+
switch (action->tag)
{
case spawn_do_close:
@@ -233,6 +288,11 @@ __spawni_child (void *arguments)
break;
case spawn_do_dup2:
+ if (use_pipe && action->action.dup2_action.fd == args->pipe[1])
+ {
+ errno = EBADF;
+ goto fail;
+ }
/* Austin Group issue #411 requires adddup2 action with source
and destination being equal to remove close-on-exec flag. */
if (action->action.dup2_action.fd
@@ -264,8 +324,16 @@ __spawni_child (void *arguments)
case spawn_do_closefrom:
{
int lowfd = action->action.closefrom_action.from;
- int r = INLINE_SYSCALL_CALL (close_range, lowfd, ~0U, 0);
- if (r != 0 && !__closefrom_fallback (lowfd, false))
+ /* Skip the pipe descriptor if it is used. No need to handle
+ it since it created and duplicated with O_CLOEXEC. */
+ if (use_pipe
+ && args->pipe[1] > action->action.closefrom_action.from)
+ {
+ if (!spawni_fa_closerange (lowfd, args->pipe[1] - 1)
+ || !spawni_fa_closerange (args->pipe[1] + 1, ~0U))
+ goto fail;
+ }
+ else if (!spawni_fa_closerange (lowfd, ~0U))
goto fail;
} break;
@@ -300,10 +368,114 @@ fail:
(EINTERNALBUG) describing that, use ECHILD. Another option would
be to set args->err to some negative sentinel and have the parent
abort(), but that seems needlessly harsh. */
- args->err = errno ? : ECHILD;
+ int ret = errno ? : ECHILD;
+ if (use_pipe)
+ {
+ while (__write_nocancel (args->pipe[1], &ret, sizeof (ret)) < 0)
+ if (errno == EPIPE || errno == EBADF)
+ break;
+ }
+ else
+ args->err = ret;
+
_exit (SPAWN_ERROR);
}
+static pid_t
+clone_call (struct posix_spawn_args *args, int flags, void *stack,
+ size_t stack_size)
+{
+ struct clone_args clone_args =
+ {
+ .flags = flags,
+ .exit_signal = SIGCHLD,
+ .stack = (uintptr_t) stack,
+ .stack_size = stack_size,
+ };
+ return __clone_internal (&clone_args, spawni_child, args);
+}
+
+/* Spawn a new process using clone with CLONE_VM | CLONE_VFORK (to optimize
+ memory and overcommit) and return TRUE if the helper was created or if the
+ failure was not due resource exhaustion. */
+static bool
+spawni_clone (struct posix_spawn_args *args, pid_t *new_pid, int *ec,
+ void *stack, size_t stack_size)
+{
+ /* The clone flags used will create a new child that will run in the same
+ memory space (CLONE_VM) and the execution of calling thread will be
+ suspend until the child calls execve or _exit.
+
+ Also since the calling thread execution will be suspend, there is not
+ need for CLONE_SETTLS. Although parent and child share the same TLS
+ namespace, there will be no concurrent access for TLS variables (errno
+ for instance). */
+ *new_pid = clone_call (args, CLONE_VM | CLONE_VFORK, stack, stack_size);
+
+ /* It needs to collect the case where the auxiliary process was created
+ but failed to execute the file (due either any preparation step or
+ for execve itself). */
+ if (*new_pid > 0)
+ {
+ /* Also, it handles the unlikely case where the auxiliary process was
+ terminated before calling execve as if it was successfully. The
+ args.err is set to 0 as default and changed to a positive value
+ only in case of failure, so in case of premature termination
+ due a signal args.err will remain zeroed and it will be up to
+ caller to actually collect it. */
+ *ec = args->err;
+ if (*ec > 0)
+ /* There still an unlikely case where the child is cancelled after
+ setting args.err, due to a positive error value. Also there is
+ possible pid reuse race (where the kernel allocated the same pid
+ to an unrelated process). Unfortunately due synchronization
+ issues where the kernel might not have the process collected
+ the waitpid below can not use WNOHANG. */
+ __waitpid (*new_pid, NULL, 0);
+ }
+ else
+ *ec = errno;
+
+ /* There is no much point in retrying with fork and exec if kernel returns a
+ failure due resource exhaustion. */
+ return *new_pid > 0 || (errno == ENOMEM || errno == EAGAIN);
+}
+
+/* Fallback spawn case which does not use CLONE_VM. Any prepration step or
+ execve failure is passed with a pipe, which requires additional care by
+ the helper stating process since it additional file descriptors handle. */
+static void
+spawni_vfork (struct posix_spawn_args *args, pid_t *new_pid, int *ec,
+ char *stack, size_t stack_size)
+{
+ if (__pipe2 (args->pipe, O_CLOEXEC) != 0)
+ {
+ *ec = errno;
+ return;
+ }
+
+ /* Do not trigger atfork handler nor any internal state reset since the
+ helper process will call execve. */
+ *new_pid = clone_call (args, CLONE_VFORK, stack, stack_size);
+ if (*new_pid == 0)
+ spawni_child (args);
+
+ __close (args->pipe[1]);
+
+ if (*new_pid > 0)
+ {
+ if (__read (args->pipe[0], ec, sizeof *ec) != sizeof *ec)
+ /* A successful execve will close the helper process pipe end. */
+ *ec = 0;
+ else
+ __waitpid (*new_pid, NULL, 0);
+ }
+ else
+ *ec = errno;
+
+ __close (args->pipe[0]);
+}
+
/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
Before running the process perform the actions described in FILE-ACTIONS. */
static int
@@ -367,49 +539,16 @@ __spawnix (pid_t * pid, const char *file,
args.argc = argc;
args.envp = envp;
args.xflags = xflags;
+ args.pipe[0] = args.pipe[1] = -1;
__libc_signal_block_all (&args.oldmask);
- /* The clone flags used will create a new child that will run in the same
- memory space (CLONE_VM) and the execution of calling thread will be
- suspend until the child calls execve or _exit.
-
- Also since the calling thread execution will be suspend, there is not
- need for CLONE_SETTLS. Although parent and child share the same TLS
- namespace, there will be no concurrent access for TLS variables (errno
- for instance). */
- struct clone_args clone_args =
- {
- .flags = CLONE_VM | CLONE_VFORK,
- .exit_signal = SIGCHLD,
- .stack = (uintptr_t) stack,
- .stack_size = stack_size,
- };
- new_pid = __clone_internal (&clone_args, __spawni_child, &args);
-
- /* It needs to collect the case where the auxiliary process was created
- but failed to execute the file (due either any preparation step or
- for execve itself). */
- if (new_pid > 0)
- {
- /* Also, it handles the unlikely case where the auxiliary process was
- terminated before calling execve as if it was successfully. The
- args.err is set to 0 as default and changed to a positive value
- only in case of failure, so in case of premature termination
- due a signal args.err will remain zeroed and it will be up to
- caller to actually collect it. */
- ec = args.err;
- if (ec > 0)
- /* There still an unlikely case where the child is cancelled after
- setting args.err, due to a positive error value. Also there is
- possible pid reuse race (where the kernel allocated the same pid
- to an unrelated process). Unfortunately due synchronization
- issues where the kernel might not have the process collected
- the waitpid below can not use WNOHANG. */
- __waitpid (new_pid, NULL, 0);
- }
- else
- ec = errno;
+ /* clone with CLONE_VM | CLONE_VFORK may fail for some namespace restriction
+ (for instance Linux does not allow processes in different time namespaces
+ to share address space) and in this case clone fails with EINVAL. Retry
+ with fork and exec. */
+ if (!spawni_clone (&args, &new_pid, &ec, stack, stack_size))
+ spawni_vfork (&args, &new_pid, &ec, stack, stack_size);
__munmap (stack, stack_size);