Patchwork Fix amd64->i386 linux syscall restart problem

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Submitter Kevin Buettner
Date March 17, 2019, 5:13 a.m.
Message ID <20190316221341.021f7c62@f29-4.lan>
Download mbox | patch
Permalink /patch/31872/
State New
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Comments

Kevin Buettner - March 17, 2019, 5:13 a.m.
This commit fixes some failures in gdb.base/interrupt.exp
when debugging a 32-bit i386 linux inferior from an amd64 host.

When running the following test...

  make check RUNTESTFLAGS="--target_board unix/-m32 interrupt.exp"

... without this commit, I see the following output:

FAIL: gdb.base/interrupt.exp: continue (the program exited)
FAIL: gdb.base/interrupt.exp: echo data
FAIL: gdb.base/interrupt.exp: Send Control-C, second time
FAIL: gdb.base/interrupt.exp: signal SIGINT (the program is no longer running)
ERROR: Undefined command "".
ERROR: GDB process no longer exists

		=== gdb Summary ===

When the test is run with this commit in place, we see 12 passes
instead.  This is the desired behavior.

Analysis:

On Linux, when a syscall is interrupted by a signal, the syscall
may return -ERESTARTSYS when a signal occurs.  Doing so indicates that
the syscall is restartable.  Then, depending on settings associated
with the signal handler, and after the signal handler is called, the
kernel can then either return -EINTR or can cause the syscall to be
restarted.  In this discussion, we are concerned with the latter
case.

On i386, the kernel returns this status via the EAX register.

When debugging a 32-bit (i386) process from a 64-bit (amd64)
GDB, the debugger fetches 64-bit registers even though the
process being debugged is 32-bit.  Since we're debugging a 32-bit
target, only 32 bits are being saved in the register cache.
Now, ideally, GDB would save all 64-bits in the regcache and
then would be able to restore those same values when it comes
time to continue the target.  I've looked into doing this, but
it's not easy and I don't see many benefits to doing so.  One
benefit, however, would be that EAX would appear as a negative
value for doing syscall restarts.

At the moment, GDB is setting the high 32 bits of RAX (and other
registers too) to 0.  So, when GDB restores EAX just prior to
a syscall restart, the high 32 bits of RAX are zeroed, thus making
it look like a positive value.  For this particular purpose, we
need to sign extend EAX so that RAX will appear as a negative
value when EAX is set to -ERESTARTSYS.  This in turn will cause
the signal handling code in the kernel to recognize -ERESTARTSYS
which will in turn cause the syscall to be restarted.

This commit is based on work by Jan Kratochvil from 2009:

https://sourceware.org/ml/gdb-patches/2009-11/msg00592.html

Jan's patch had the sign extension code in amd64-nat.c.  Several
other native targets make use of this code, so it seemed better
to move the sign extension code to a linux specific file.  I
also added similar code to gdbserver.

Another approach is to fix the problem in the kernel.  Hui Zhu
tried to get a fix into the kernel back in 2014, but it was not
accepted.  Discussion regarding this approach may be found here:

https://lore.kernel.org/patchwork/patch/457841/

Even if a fix were to be put into the kernel, we'd still need
some kind of fix in GDB in order to support older kernels.

Finally, I'll note that Fedora has been carrying a similar patch for
at least nine years.  Other distributions, including RHEL and CentOS
have picked up this change and have been using it too.

gdb/ChangeLog:

	* amd64-linux-nat.c (amd64_linux_collect_native_gregset): New
	function.
	(fill_gregset): Call amd64_linux_collect_native_gregset instead
	of amd64_collect_native_gregset.
	(amd64_linux_nat_target::store_registers): Likewise.

gdb/gdbserver/ChangeLog:

	* linux-x86-low.c (x86_fill_gregset): Sign extend EAX value
	when using a 64-bit gdbserver.
---
 gdb/amd64-linux-nat.c         | 32 ++++++++++++++++++++++++++++++--
 gdb/gdbserver/linux-x86-low.c |  9 +++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)
Kevin Buettner - April 4, 2019, 4:51 p.m.
Ping.

On Sat, 16 Mar 2019 22:13:41 -0700
Kevin Buettner <kevinb@redhat.com> wrote:

> This commit fixes some failures in gdb.base/interrupt.exp
> when debugging a 32-bit i386 linux inferior from an amd64 host.
> 
> When running the following test...
> 
>   make check RUNTESTFLAGS="--target_board unix/-m32 interrupt.exp"
> 
> ... without this commit, I see the following output:
> 
> FAIL: gdb.base/interrupt.exp: continue (the program exited)
> FAIL: gdb.base/interrupt.exp: echo data
> FAIL: gdb.base/interrupt.exp: Send Control-C, second time
> FAIL: gdb.base/interrupt.exp: signal SIGINT (the program is no longer running)
> ERROR: Undefined command "".
> ERROR: GDB process no longer exists
> 
> 		=== gdb Summary ===
> 
> When the test is run with this commit in place, we see 12 passes
> instead.  This is the desired behavior.
> 
> Analysis:
> 
> On Linux, when a syscall is interrupted by a signal, the syscall
> may return -ERESTARTSYS when a signal occurs.  Doing so indicates that
> the syscall is restartable.  Then, depending on settings associated
> with the signal handler, and after the signal handler is called, the
> kernel can then either return -EINTR or can cause the syscall to be
> restarted.  In this discussion, we are concerned with the latter
> case.
> 
> On i386, the kernel returns this status via the EAX register.
> 
> When debugging a 32-bit (i386) process from a 64-bit (amd64)
> GDB, the debugger fetches 64-bit registers even though the
> process being debugged is 32-bit.  Since we're debugging a 32-bit
> target, only 32 bits are being saved in the register cache.
> Now, ideally, GDB would save all 64-bits in the regcache and
> then would be able to restore those same values when it comes
> time to continue the target.  I've looked into doing this, but
> it's not easy and I don't see many benefits to doing so.  One
> benefit, however, would be that EAX would appear as a negative
> value for doing syscall restarts.
> 
> At the moment, GDB is setting the high 32 bits of RAX (and other
> registers too) to 0.  So, when GDB restores EAX just prior to
> a syscall restart, the high 32 bits of RAX are zeroed, thus making
> it look like a positive value.  For this particular purpose, we
> need to sign extend EAX so that RAX will appear as a negative
> value when EAX is set to -ERESTARTSYS.  This in turn will cause
> the signal handling code in the kernel to recognize -ERESTARTSYS
> which will in turn cause the syscall to be restarted.
> 
> This commit is based on work by Jan Kratochvil from 2009:
> 
> https://sourceware.org/ml/gdb-patches/2009-11/msg00592.html
> 
> Jan's patch had the sign extension code in amd64-nat.c.  Several
> other native targets make use of this code, so it seemed better
> to move the sign extension code to a linux specific file.  I
> also added similar code to gdbserver.
> 
> Another approach is to fix the problem in the kernel.  Hui Zhu
> tried to get a fix into the kernel back in 2014, but it was not
> accepted.  Discussion regarding this approach may be found here:
> 
> https://lore.kernel.org/patchwork/patch/457841/
> 
> Even if a fix were to be put into the kernel, we'd still need
> some kind of fix in GDB in order to support older kernels.
> 
> Finally, I'll note that Fedora has been carrying a similar patch for
> at least nine years.  Other distributions, including RHEL and CentOS
> have picked up this change and have been using it too.
> 
> gdb/ChangeLog:
> 
> 	* amd64-linux-nat.c (amd64_linux_collect_native_gregset): New
> 	function.
> 	(fill_gregset): Call amd64_linux_collect_native_gregset instead
> 	of amd64_collect_native_gregset.
> 	(amd64_linux_nat_target::store_registers): Likewise.
> 
> gdb/gdbserver/ChangeLog:
> 
> 	* linux-x86-low.c (x86_fill_gregset): Sign extend EAX value
> 	when using a 64-bit gdbserver.
> ---
>  gdb/amd64-linux-nat.c         | 32 ++++++++++++++++++++++++++++++--
>  gdb/gdbserver/linux-x86-low.c |  9 +++++++++
>  2 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c
> index a0bb105f5a..06018c8f1c 100644
> --- a/gdb/amd64-linux-nat.c
> +++ b/gdb/amd64-linux-nat.c
> @@ -92,6 +92,34 @@ static int amd64_linux_gregset32_reg_offset[] =
>  /* Transfering the general-purpose registers between GDB, inferiors
>     and core files.  */
>  
> +/* See amd64_collect_native_gregset.  This linux specific version handles
> +   issues with negative EAX values not being restored correctly upon syscall
> +   return when debugging 32-bit targets.  It has no effect on 64-bit
> +   targets.  */
> +
> +static void
> +amd64_linux_collect_native_gregset (const struct regcache *regcache,
> +			            void *gregs, int regnum)
> +{
> +  amd64_collect_native_gregset (regcache, gregs, regnum);
> +
> +  struct gdbarch *gdbarch = regcache->arch ();
> +  if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32)
> +    {
> +      /* Sign-extend %eax as during return from a syscall it is being checked
> +	 for -ERESTART* values -512 being above 0xfffffffffffffe00; tested by
> +	 interrupt.exp.	 */
> +
> +      if (regnum == -1 || regnum == I386_EAX_REGNUM)
> +	{
> +	  void *ptr = (gdb_byte *) gregs 
> +		    + amd64_linux_gregset32_reg_offset[I386_EAX_REGNUM];
> +
> +	  *(int64_t *) ptr = *(int32_t *) ptr;
> +	}
> +    }
> +}
> +
>  /* Fill GDB's register cache with the general-purpose register values
>     in *GREGSETP.  */
>  
> @@ -109,7 +137,7 @@ void
>  fill_gregset (const struct regcache *regcache,
>  	      elf_gregset_t *gregsetp, int regnum)
>  {
> -  amd64_collect_native_gregset (regcache, gregsetp, regnum);
> +  amd64_linux_collect_native_gregset (regcache, gregsetp, regnum);
>  }
>  
>  /* Transfering floating-point registers between GDB, inferiors and cores.  */
> @@ -237,7 +265,7 @@ amd64_linux_nat_target::store_registers (struct regcache *regcache, int regnum)
>        if (ptrace (PTRACE_GETREGS, tid, 0, (long) &regs) < 0)
>  	perror_with_name (_("Couldn't get registers"));
>  
> -      amd64_collect_native_gregset (regcache, &regs, regnum);
> +      amd64_linux_collect_native_gregset (regcache, &regs, regnum);
>  
>        if (ptrace (PTRACE_SETREGS, tid, 0, (long) &regs) < 0)
>  	perror_with_name (_("Couldn't write registers"));
> diff --git a/gdb/gdbserver/linux-x86-low.c b/gdb/gdbserver/linux-x86-low.c
> index 029796e361..20f369c496 100644
> --- a/gdb/gdbserver/linux-x86-low.c
> +++ b/gdb/gdbserver/linux-x86-low.c
> @@ -338,6 +338,15 @@ x86_fill_gregset (struct regcache *regcache, void *buf)
>  
>    collect_register_by_name (regcache, "orig_eax",
>  			    ((char *) buf) + ORIG_EAX * REGSIZE);
> +
> +  /* Sign extend EAX value to avoid potential syscall restart problems.  */
> +  if (register_size (regcache->tdesc, 0) == 4)
> +    {
> +      void *ptr = (gdb_byte *) buf
> +                + i386_regmap[find_regno (regcache->tdesc, "eax")];
> +
> +      *(int64_t *) ptr = *(int32_t *) ptr;
> +    }
>  }
>  
>  static void
>
Pedro Alves - April 9, 2019, 5:46 p.m.
On 3/17/19 5:13 AM, Kevin Buettner wrote:
> This commit fixes some failures in gdb.base/interrupt.exp
> when debugging a 32-bit i386 linux inferior from an amd64 host.
> 
> When running the following test...
> 
>   make check RUNTESTFLAGS="--target_board unix/-m32 interrupt.exp"
> 
> ... without this commit, I see the following output:
> 
> FAIL: gdb.base/interrupt.exp: continue (the program exited)
> FAIL: gdb.base/interrupt.exp: echo data
> FAIL: gdb.base/interrupt.exp: Send Control-C, second time
> FAIL: gdb.base/interrupt.exp: signal SIGINT (the program is no longer running)
> ERROR: Undefined command "".
> ERROR: GDB process no longer exists
> 
> 		=== gdb Summary ===
> 
> When the test is run with this commit in place, we see 12 passes
> instead.  This is the desired behavior.
> 
> Analysis:
> 
> On Linux, when a syscall is interrupted by a signal, the syscall
> may return -ERESTARTSYS when a signal occurs.  Doing so indicates that
> the syscall is restartable.  Then, depending on settings associated
> with the signal handler, and after the signal handler is called, the
> kernel can then either return -EINTR or can cause the syscall to be
> restarted.  In this discussion, we are concerned with the latter
> case.
> 
> On i386, the kernel returns this status via the EAX register.
> 
> When debugging a 32-bit (i386) process from a 64-bit (amd64)
> GDB, the debugger fetches 64-bit registers even though the
> process being debugged is 32-bit.  Since we're debugging a 32-bit
> target, only 32 bits are being saved in the register cache.
> Now, ideally, GDB would save all 64-bits in the regcache and
> then would be able to restore those same values when it comes
> time to continue the target.  I've looked into doing this, but
> it's not easy and I don't see many benefits to doing so.  

Yeah, it'd not be easy.  We'd have to make the target backend
report a 64-bit target description, and then make gdb expose a
32-bit view over the registers, using pseudo registers.  MIPS
port has something like that (mips64_transfers_32bit_regs_p),
though, so there's precedent.  An advantage in such a design is that 
would fix the troubles with debugging low level x86 code that changes
machine mode (16-bit/32-bit/64-bit), without losing any of the
high bits in the registers, which are also preserved by the cpu.
We could then even have a gdb knob to manually switch "view"
mode (16-bit/32-bit/64-bit), and that would not change how the
registers are transferred in the backend -- it would always work
at the max width the machine supports.  That is, an advantage, compared
to a solution that makes gdb fetch a new target description when
the mode changes.

> One
> benefit, however, would be that EAX would appear as a negative
> value for doing syscall restarts.
> 
> At the moment, GDB is setting the high 32 bits of RAX (and other
> registers too) to 0.  So, when GDB restores EAX just prior to
> a syscall restart, the high 32 bits of RAX are zeroed, thus making
> it look like a positive value.  For this particular purpose, we
> need to sign extend EAX so that RAX will appear as a negative
> value when EAX is set to -ERESTARTSYS.  This in turn will cause
> the signal handling code in the kernel to recognize -ERESTARTSYS
> which will in turn cause the syscall to be restarted.
> 
> This commit is based on work by Jan Kratochvil from 2009:
> 
> https://sourceware.org/ml/gdb-patches/2009-11/msg00592.html
> 
> Jan's patch had the sign extension code in amd64-nat.c.  Several
> other native targets make use of this code, so it seemed better
> to move the sign extension code to a linux specific file.  I
> also added similar code to gdbserver.
> 
> Another approach is to fix the problem in the kernel.  Hui Zhu
> tried to get a fix into the kernel back in 2014, but it was not
> accepted.  Discussion regarding this approach may be found here:
> 
> https://lore.kernel.org/patchwork/patch/457841/
> 

In that discussion, I proposed a different kernel fix, and 
H. Peter Anvin kind of agreed with it (said "This seems a lot saner"),
but no one ever submitted such a patch, I believe.

> Even if a fix were to be put into the kernel, we'd still need
> some kind of fix in GDB in order to support older kernels.

That's reasonable.

> 
> Finally, I'll note that Fedora has been carrying a similar patch for
> at least nine years.  Other distributions, including RHEL and CentOS
> have picked up this change and have been using it too.
> 
> gdb/ChangeLog:
> 
> 	* amd64-linux-nat.c (amd64_linux_collect_native_gregset): New
> 	function.
> 	(fill_gregset): Call amd64_linux_collect_native_gregset instead
> 	of amd64_collect_native_gregset.
> 	(amd64_linux_nat_target::store_registers): Likewise.
> 
> gdb/gdbserver/ChangeLog:
> 
> 	* linux-x86-low.c (x86_fill_gregset): Sign extend EAX value
> 	when using a 64-bit gdbserver.
> ---
>  gdb/amd64-linux-nat.c         | 32 ++++++++++++++++++++++++++++++--
>  gdb/gdbserver/linux-x86-low.c |  9 +++++++++
>  2 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c
> index a0bb105f5a..06018c8f1c 100644
> --- a/gdb/amd64-linux-nat.c
> +++ b/gdb/amd64-linux-nat.c
> @@ -92,6 +92,34 @@ static int amd64_linux_gregset32_reg_offset[] =
>  /* Transfering the general-purpose registers between GDB, inferiors
>     and core files.  */
>  
> +/* See amd64_collect_native_gregset.  This linux specific version handles
> +   issues with negative EAX values not being restored correctly upon syscall
> +   return when debugging 32-bit targets.  It has no effect on 64-bit
> +   targets.  */
> +
> +static void
> +amd64_linux_collect_native_gregset (const struct regcache *regcache,
> +			            void *gregs, int regnum)
> +{
> +  amd64_collect_native_gregset (regcache, gregs, regnum);
> +
> +  struct gdbarch *gdbarch = regcache->arch ();
> +  if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32)
> +    {
> +      /* Sign-extend %eax as during return from a syscall it is being checked
> +	 for -ERESTART* values -512 being above 0xfffffffffffffe00; tested by
> +	 interrupt.exp.	 */> +
> +      if (regnum == -1 || regnum == I386_EAX_REGNUM)
> +	{
> +	  void *ptr = (gdb_byte *) gregs 
> +		    + amd64_linux_gregset32_reg_offset[I386_EAX_REGNUM];

Need to wrap the expression that spawns two lines in ()s.

> +
> +	  *(int64_t *) ptr = *(int32_t *) ptr;
> +	}
> +    }
> +}
> +
>  /* Fill GDB's register cache with the general-purpose register values
>     in *GREGSETP.  */
>  
> @@ -109,7 +137,7 @@ void
>  fill_gregset (const struct regcache *regcache,
>  	      elf_gregset_t *gregsetp, int regnum)
>  {
> -  amd64_collect_native_gregset (regcache, gregsetp, regnum);
> +  amd64_linux_collect_native_gregset (regcache, gregsetp, regnum);
>  }
>  
>  /* Transfering floating-point registers between GDB, inferiors and cores.  */
> @@ -237,7 +265,7 @@ amd64_linux_nat_target::store_registers (struct regcache *regcache, int regnum)
>        if (ptrace (PTRACE_GETREGS, tid, 0, (long) &regs) < 0)
>  	perror_with_name (_("Couldn't get registers"));
>  
> -      amd64_collect_native_gregset (regcache, &regs, regnum);
> +      amd64_linux_collect_native_gregset (regcache, &regs, regnum);
>  
>        if (ptrace (PTRACE_SETREGS, tid, 0, (long) &regs) < 0)
>  	perror_with_name (_("Couldn't write registers"));
> diff --git a/gdb/gdbserver/linux-x86-low.c b/gdb/gdbserver/linux-x86-low.c
> index 029796e361..20f369c496 100644
> --- a/gdb/gdbserver/linux-x86-low.c
> +++ b/gdb/gdbserver/linux-x86-low.c
> @@ -338,6 +338,15 @@ x86_fill_gregset (struct regcache *regcache, void *buf)
>  
>    collect_register_by_name (regcache, "orig_eax",
>  			    ((char *) buf) + ORIG_EAX * REGSIZE);
> +
> +  /* Sign extend EAX value to avoid potential syscall restart problems.  */

I'd rather see both implementations use the same comment.  Could you
merge them?

> +  if (register_size (regcache->tdesc, 0) == 4)
> +    {
> +      void *ptr = (gdb_byte *) buf
> +                + i386_regmap[find_regno (regcache->tdesc, "eax")];

Ditto wrt parens.

> +
> +      *(int64_t *) ptr = *(int32_t *) ptr;
> +    }
>  }
>  
>  static void
> 

Otherwise OK.

Thanks,
Pedro Alves

Patch

diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c
index a0bb105f5a..06018c8f1c 100644
--- a/gdb/amd64-linux-nat.c
+++ b/gdb/amd64-linux-nat.c
@@ -92,6 +92,34 @@  static int amd64_linux_gregset32_reg_offset[] =
 /* Transfering the general-purpose registers between GDB, inferiors
    and core files.  */
 
+/* See amd64_collect_native_gregset.  This linux specific version handles
+   issues with negative EAX values not being restored correctly upon syscall
+   return when debugging 32-bit targets.  It has no effect on 64-bit
+   targets.  */
+
+static void
+amd64_linux_collect_native_gregset (const struct regcache *regcache,
+			            void *gregs, int regnum)
+{
+  amd64_collect_native_gregset (regcache, gregs, regnum);
+
+  struct gdbarch *gdbarch = regcache->arch ();
+  if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32)
+    {
+      /* Sign-extend %eax as during return from a syscall it is being checked
+	 for -ERESTART* values -512 being above 0xfffffffffffffe00; tested by
+	 interrupt.exp.	 */
+
+      if (regnum == -1 || regnum == I386_EAX_REGNUM)
+	{
+	  void *ptr = (gdb_byte *) gregs 
+		    + amd64_linux_gregset32_reg_offset[I386_EAX_REGNUM];
+
+	  *(int64_t *) ptr = *(int32_t *) ptr;
+	}
+    }
+}
+
 /* Fill GDB's register cache with the general-purpose register values
    in *GREGSETP.  */
 
@@ -109,7 +137,7 @@  void
 fill_gregset (const struct regcache *regcache,
 	      elf_gregset_t *gregsetp, int regnum)
 {
-  amd64_collect_native_gregset (regcache, gregsetp, regnum);
+  amd64_linux_collect_native_gregset (regcache, gregsetp, regnum);
 }
 
 /* Transfering floating-point registers between GDB, inferiors and cores.  */
@@ -237,7 +265,7 @@  amd64_linux_nat_target::store_registers (struct regcache *regcache, int regnum)
       if (ptrace (PTRACE_GETREGS, tid, 0, (long) &regs) < 0)
 	perror_with_name (_("Couldn't get registers"));
 
-      amd64_collect_native_gregset (regcache, &regs, regnum);
+      amd64_linux_collect_native_gregset (regcache, &regs, regnum);
 
       if (ptrace (PTRACE_SETREGS, tid, 0, (long) &regs) < 0)
 	perror_with_name (_("Couldn't write registers"));
diff --git a/gdb/gdbserver/linux-x86-low.c b/gdb/gdbserver/linux-x86-low.c
index 029796e361..20f369c496 100644
--- a/gdb/gdbserver/linux-x86-low.c
+++ b/gdb/gdbserver/linux-x86-low.c
@@ -338,6 +338,15 @@  x86_fill_gregset (struct regcache *regcache, void *buf)
 
   collect_register_by_name (regcache, "orig_eax",
 			    ((char *) buf) + ORIG_EAX * REGSIZE);
+
+  /* Sign extend EAX value to avoid potential syscall restart problems.  */
+  if (register_size (regcache->tdesc, 0) == 4)
+    {
+      void *ptr = (gdb_byte *) buf
+                + i386_regmap[find_regno (regcache->tdesc, "eax")];
+
+      *(int64_t *) ptr = *(int32_t *) ptr;
+    }
 }
 
 static void