diff mbox

binfmt_misc: pass binfmt_misc P flag to the interpreter

Message ID 20200306080905.173466-1-syq@debian.org
State New
Headers show

Commit Message

YunQiang Su March 6, 2020, 8:09 a.m. UTC
From: Laurent Vivier <laurent@vivier.eu>

It can be useful to the interpreter to know which flags are in use.

For instance, knowing if the preserve-argv[0] is in use would
allow to skip the pathname argument.

This patch uses an unused auxiliary vector, AT_FLAGS, to add a
flag to inform interpreter if the preserve-argv[0] is enabled.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: YunQiang Su <syq@debian.org>
---

Notes:
    This patch uses AT_FLAGS, which has never been used. We need to make a decision.
    since it is about the fundmental API of userland.

    This can be tested with QEMU from my branch:

      https://github.com/vivier/qemu/commits/binfmt-argv0

    With something like:

      # cp ..../qemu-ppc /chroot/powerpc/jessie

      # qemu-binfmt-conf.sh --qemu-path / --systemd ppc --credential yes \
                            --persistent no --preserve-argv0 yes
      # systemctl restart systemd-binfmt.service
      # cat /proc/sys/fs/binfmt_misc/qemu-ppc
      enabled
      interpreter //qemu-ppc
      flags: POC
      offset 0
      magic 7f454c4601020100000000000000000000020014
      mask ffffffffffffff00fffffffffffffffffffeffff
      # chroot /chroot/powerpc/jessie  sh -c 'echo $0'
      sh

      # qemu-binfmt-conf.sh --qemu-path / --systemd ppc --credential yes \
                            --persistent no --preserve-argv0 no
      # systemctl restart systemd-binfmt.service
      # cat /proc/sys/fs/binfmt_misc/qemu-ppc
      enabled
      interpreter //qemu-ppc
      flags: OC
      offset 0
      magic 7f454c4601020100000000000000000000020014
      mask ffffffffffffff00fffffffffffffffffffeffff
      # chroot /chroot/powerpc/jessie  sh -c 'echo $0'
      /bin/sh

    v4: Update to merge with linux-next: 20200305.
    v3: mix my patch with one from YunQiang Su and my comments on it
        introduce a new flag in the uabi for the AT_FLAGS
    v2: only pass special flags (remove Magic and Enabled flags)
---
 fs/binfmt_elf.c              | 6 ++++--
 fs/binfmt_elf_fdpic.c        | 5 ++++-
 fs/binfmt_misc.c             | 4 +++-
 include/linux/binfmts.h      | 4 ++++
 include/uapi/linux/binfmts.h | 4 ++++
 5 files changed, 19 insertions(+), 4 deletions(-)

Comments

Florian Weimer March 6, 2020, 8:13 a.m. UTC | #1
* YunQiang Su:

> +	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
> +		flags |= AT_FLAGS_PRESERVE_ARGV0;
> +	NEW_AUX_ENT(AT_FLAGS, flags);

Is it necessary to reuse AT_FLAGS?  I think it's cleaner to define a
separate AT_ tag dedicated to binfmt_misc.
Laurent Vivier March 6, 2020, 8:21 a.m. UTC | #2
Le 06/03/2020 à 09:13, Florian Weimer a écrit :
> * YunQiang Su:
> 
>> +	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
>> +		flags |= AT_FLAGS_PRESERVE_ARGV0;
>> +	NEW_AUX_ENT(AT_FLAGS, flags);
> 
> Is it necessary to reuse AT_FLAGS?  I think it's cleaner to define a
> separate AT_ tag dedicated to binfmt_misc.
> 

Not necessary, but it seemed simpler and cleaner to re-use a flag that
is marked as unused and with a name matching the new role. It avoids to
patch other packages (like glibc) to add it as it is already defined.

Thanks,
Laurent
Florian Weimer March 6, 2020, 8:37 a.m. UTC | #3
* Laurent Vivier:

> Le 06/03/2020 à 09:13, Florian Weimer a écrit :
>> * YunQiang Su:
>> 
>>> +	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
>>> +		flags |= AT_FLAGS_PRESERVE_ARGV0;
>>> +	NEW_AUX_ENT(AT_FLAGS, flags);
>> 
>> Is it necessary to reuse AT_FLAGS?  I think it's cleaner to define a
>> separate AT_ tag dedicated to binfmt_misc.
>
> Not necessary, but it seemed simpler and cleaner to re-use a flag that
> is marked as unused and with a name matching the new role. It avoids to
> patch other packages (like glibc) to add it as it is already defined.

You still need to define AT_FLAGS_PRESERVE_ARGV0.  At that point, you
might as well define AT_BINFMT and AT_BINFMT_PRESERVE_ARGV0.
Laurent Vivier March 6, 2020, 11:13 a.m. UTC | #4
Le 06/03/2020 à 09:37, Florian Weimer a écrit :
> * Laurent Vivier:
> 
>> Le 06/03/2020 à 09:13, Florian Weimer a écrit :
>>> * YunQiang Su:
>>>
>>>> +	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
>>>> +		flags |= AT_FLAGS_PRESERVE_ARGV0;
>>>> +	NEW_AUX_ENT(AT_FLAGS, flags);
>>>
>>> Is it necessary to reuse AT_FLAGS?  I think it's cleaner to define a
>>> separate AT_ tag dedicated to binfmt_misc.
>>
>> Not necessary, but it seemed simpler and cleaner to re-use a flag that
>> is marked as unused and with a name matching the new role. It avoids to
>> patch other packages (like glibc) to add it as it is already defined.
> 
> You still need to define AT_FLAGS_PRESERVE_ARGV0.  At that point, you
> might as well define AT_BINFMT and AT_BINFMT_PRESERVE_ARGV0.
> 

Yes, you're right.

But is there any reason to not reuse AT_FLAGS?

Thanks,
Laurent
YunQiang Su March 6, 2020, 11:29 a.m. UTC | #5
Laurent Vivier <laurent@vivier.eu> 于2020年3月6日周五 下午7:13写道:
>
> Le 06/03/2020 à 09:37, Florian Weimer a écrit :
> > * Laurent Vivier:
> >
> >> Le 06/03/2020 à 09:13, Florian Weimer a écrit :
> >>> * YunQiang Su:
> >>>
> >>>> +  if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
> >>>> +          flags |= AT_FLAGS_PRESERVE_ARGV0;
> >>>> +  NEW_AUX_ENT(AT_FLAGS, flags);
> >>>
> >>> Is it necessary to reuse AT_FLAGS?  I think it's cleaner to define a
> >>> separate AT_ tag dedicated to binfmt_misc.
> >>
> >> Not necessary, but it seemed simpler and cleaner to re-use a flag that
> >> is marked as unused and with a name matching the new role. It avoids to
> >> patch other packages (like glibc) to add it as it is already defined.
> >
> > You still need to define AT_FLAGS_PRESERVE_ARGV0.  At that point, you
> > might as well define AT_BINFMT and AT_BINFMT_PRESERVE_ARGV0.
> >
>
> Yes, you're right.
>
> But is there any reason to not reuse AT_FLAGS?

AT_* only has 32 slot and now. I was afraid that maybe we shouldn't take one.
   /* AT_* values 18 through 22 are reserved */
   27,28,29,30 are not used now.
Which should we use?

>
> Thanks,
> Laurent
Florian Weimer March 6, 2020, 11:40 a.m. UTC | #6
* YunQiang Su:

> AT_* only has 32 slot and now. I was afraid that maybe we shouldn't take one.
>    /* AT_* values 18 through 22 are reserved */
>    27,28,29,30 are not used now.
> Which should we use?

Where does this limit of 32 tags come from?  I don't see it from a
userspace perspective.
YunQiang Su March 6, 2020, 11:48 a.m. UTC | #7
Florian Weimer <fw@deneb.enyo.de> 于2020年3月6日周五 下午7:42写道:
>
> * YunQiang Su:
>
> > AT_* only has 32 slot and now. I was afraid that maybe we shouldn't take one.
> >    /* AT_* values 18 through 22 are reserved */
> >    27,28,29,30 are not used now.
> > Which should we use?
>
> Where does this limit of 32 tags come from?  I don't see it from a
> userspace perspective.

Sorry it is my mistake: In linux/auxvec.h, I saw

#define AT_RANDOM 25    /* address of 16 random bytes */
#define AT_HWCAP2 26    /* extension of AT_HWCAP */

#define AT_EXECFN  31   /* filename of program */

The number jump to 31 from 26.

It is my fault: in x86_64-linux-gnu/bits/auxv.h, the max number is 47 now.
Florian Weimer March 6, 2020, 12:07 p.m. UTC | #8
* YunQiang Su:

> Florian Weimer <fw@deneb.enyo.de> 于2020年3月6日周五 下午7:42写道:
>>
>> * YunQiang Su:
>>
>> > AT_* only has 32 slot and now. I was afraid that maybe we shouldn't take one.
>> >    /* AT_* values 18 through 22 are reserved */
>> >    27,28,29,30 are not used now.
>> > Which should we use?
>>
>> Where does this limit of 32 tags come from?  I don't see it from a
>> userspace perspective.
>
> Sorry it is my mistake: In linux/auxvec.h, I saw
>
> #define AT_RANDOM 25    /* address of 16 random bytes */
> #define AT_HWCAP2 26    /* extension of AT_HWCAP */
>
> #define AT_EXECFN  31   /* filename of program */
>
> The number jump to 31 from 26.
>
> It is my fault: in x86_64-linux-gnu/bits/auxv.h, the max number is 47 now.

So AT_* tags aren't a scarce resource after all?
Laurent Vivier March 6, 2020, 12:07 p.m. UTC | #9
Le 06/03/2020 à 12:48, YunQiang Su a écrit :
> Florian Weimer <fw@deneb.enyo.de> 于2020年3月6日周五 下午7:42写道:
>>
>> * YunQiang Su:
>>
>>> AT_* only has 32 slot and now. I was afraid that maybe we shouldn't take one.
>>>    /* AT_* values 18 through 22 are reserved */
>>>    27,28,29,30 are not used now.
>>> Which should we use?
>>
>> Where does this limit of 32 tags come from?  I don't see it from a
>> userspace perspective.
> 
> Sorry it is my mistake: In linux/auxvec.h, I saw
> 
> #define AT_RANDOM 25    /* address of 16 random bytes */
> #define AT_HWCAP2 26    /* extension of AT_HWCAP */
> 
> #define AT_EXECFN  31   /* filename of program */
> 
> The number jump to 31 from 26.
> 
> It is my fault: in x86_64-linux-gnu/bits/auxv.h, the max number is 47 now.
> 

Numbers starting from 32 are arch specific.
18 to 22 are also reserved (and used by some archs).
(linux/arch/*/include/uapi/asm/auxvec.h)

So there is only 4 entries available (27 to 30)
(linux/include/uapi/linux/auxvec.h)
Do we want to waste one more entry whereas we can use an unused one?

Thanks,
Laurent
diff mbox

Patch

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 13f25e241ac4..e9f5d135b847 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -178,7 +178,7 @@  create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	const char *k_base_platform = ELF_BASE_PLATFORM;
 	unsigned char k_rand_bytes[16];
 	int items;
-	elf_addr_t *elf_info;
+	elf_addr_t *elf_info, flags = 0;
 	int ei_index;
 	const struct cred *cred = current_cred();
 	struct vm_area_struct *vma;
@@ -253,7 +253,9 @@  create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	NEW_AUX_ENT(AT_PHENT, sizeof(struct elf_phdr));
 	NEW_AUX_ENT(AT_PHNUM, exec->e_phnum);
 	NEW_AUX_ENT(AT_BASE, interp_load_addr);
-	NEW_AUX_ENT(AT_FLAGS, 0);
+	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
+		flags |= AT_FLAGS_PRESERVE_ARGV0;
+	NEW_AUX_ENT(AT_FLAGS, flags);
 	NEW_AUX_ENT(AT_ENTRY, e_entry);
 	NEW_AUX_ENT(AT_UID, from_kuid_munged(cred->user_ns, cred->uid));
 	NEW_AUX_ENT(AT_EUID, from_kuid_munged(cred->user_ns, cred->euid));
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 240f66663543..abb90d82aa58 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -507,6 +507,7 @@  static int create_elf_fdpic_tables(struct linux_binprm *bprm,
 	char __user *u_platform, *u_base_platform, *p;
 	int loop;
 	int nr;	/* reset for each csp adjustment */
+	unsigned long flags = 0;
 
 #ifdef CONFIG_MMU
 	/* In some cases (e.g. Hyper-Threading), we want to avoid L1 evictions
@@ -647,7 +648,9 @@  static int create_elf_fdpic_tables(struct linux_binprm *bprm,
 	NEW_AUX_ENT(AT_PHENT,	sizeof(struct elf_phdr));
 	NEW_AUX_ENT(AT_PHNUM,	exec_params->hdr.e_phnum);
 	NEW_AUX_ENT(AT_BASE,	interp_params->elfhdr_addr);
-	NEW_AUX_ENT(AT_FLAGS,	0);
+	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
+		flags |= AT_FLAGS_PRESERVE_ARGV0;
+	NEW_AUX_ENT(AT_FLAGS,	flags);
 	NEW_AUX_ENT(AT_ENTRY,	exec_params->entry_addr);
 	NEW_AUX_ENT(AT_UID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->uid));
 	NEW_AUX_ENT(AT_EUID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->euid));
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index cdb45829354d..b9acdd26a654 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -154,7 +154,9 @@  static int load_misc_binary(struct linux_binprm *bprm)
 	if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
 		goto ret;
 
-	if (!(fmt->flags & MISC_FMT_PRESERVE_ARGV0)) {
+	if (fmt->flags & MISC_FMT_PRESERVE_ARGV0) {
+		bprm->interp_flags |= BINPRM_FLAGS_PRESERVE_ARGV0;
+	} else {
 		retval = remove_arg_zero(bprm);
 		if (retval)
 			goto ret;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index b40fc633f3be..265b80d5fd6f 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -78,6 +78,10 @@  struct linux_binprm {
 #define BINPRM_FLAGS_PATH_INACCESSIBLE_BIT 2
 #define BINPRM_FLAGS_PATH_INACCESSIBLE (1 << BINPRM_FLAGS_PATH_INACCESSIBLE_BIT)
 
+/* if preserve the argv0 for the interpreter  */
+#define BINPRM_FLAGS_PRESERVE_ARGV0_BIT 3
+#define BINPRM_FLAGS_PRESERVE_ARGV0 (1 << BINPRM_FLAGS_PRESERVE_ARGV0_BIT)
+
 /* Function parameter for binfmt->coredump */
 struct coredump_params {
 	const kernel_siginfo_t *siginfo;
diff --git a/include/uapi/linux/binfmts.h b/include/uapi/linux/binfmts.h
index 689025d9c185..a70747416130 100644
--- a/include/uapi/linux/binfmts.h
+++ b/include/uapi/linux/binfmts.h
@@ -18,4 +18,8 @@  struct pt_regs;
 /* sizeof(linux_binprm->buf) */
 #define BINPRM_BUF_SIZE 256
 
+/* if preserve the argv0 for the interpreter  */
+#define AT_FLAGS_PRESERVE_ARGV0_BIT 0
+#define AT_FLAGS_PRESERVE_ARGV0 (1 << AT_FLAGS_PRESERVE_ARGV0_BIT)
+
 #endif /* _UAPI_LINUX_BINFMTS_H */