* musl-libc/MIPS: detached thread exit broken since kernel commit 46e12c07b
@ 2015-06-19 2:07 Matthias Schiffer
2015-06-19 2:50 ` [musl] " Rich Felker
0 siblings, 1 reply; 4+ messages in thread
From: Matthias Schiffer @ 2015-06-19 2:07 UTC (permalink / raw
To: musl, linux-mips, linux-kernel@vger.kernel.org, Ralf Baechle
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Hi,
I've come across the issue that applications with detached threads
(using pthread_detach or a pthread_attr_t with
pthread_attr_setdetachstate) will segfault using musl-libc on MIPS as
soon as the detached thread exits. As far as I can tell, the underlying
issue is the following:
To clean up after itself, the finishing thread will call __unmapself,
which will unmap the thread's own stack and call the exit syscall
directly after that, without accessing the now unmapped stack.
This worked fine in 2012, when pthread support for MIPS was implemented
in musl. It seems to have been broken by kernel commit 46e12c07b "MIPS:
O32 / 32-bit: Always copy 4 stack arguments." (also in 2012) which made
the kernel unconditionally copy 4 stack arguments, even when the syscall
doesn't even use the arguments.
I guess this would be reasonably easy to fix up in musl, but let's also
get the linux-mips people's opinions, as that commit obviously broke the
kernel ABI...
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [musl] musl-libc/MIPS: detached thread exit broken since kernel commit 46e12c07b
2015-06-19 2:07 musl-libc/MIPS: detached thread exit broken since kernel commit 46e12c07b Matthias Schiffer
@ 2015-06-19 2:50 ` Rich Felker
2015-06-19 10:06 ` Ralf Baechle
0 siblings, 1 reply; 4+ messages in thread
From: Rich Felker @ 2015-06-19 2:50 UTC (permalink / raw
To: Matthias Schiffer
Cc: musl, linux-mips, linux-kernel@vger.kernel.org, Ralf Baechle
On Fri, Jun 19, 2015 at 04:07:52AM +0200, Matthias Schiffer wrote:
> Hi,
> I've come across the issue that applications with detached threads
> (using pthread_detach or a pthread_attr_t with
> pthread_attr_setdetachstate) will segfault using musl-libc on MIPS as
> soon as the detached thread exits. As far as I can tell, the underlying
> issue is the following:
>
> To clean up after itself, the finishing thread will call __unmapself,
> which will unmap the thread's own stack and call the exit syscall
> directly after that, without accessing the now unmapped stack.
>
> This worked fine in 2012, when pthread support for MIPS was implemented
> in musl. It seems to have been broken by kernel commit 46e12c07b "MIPS:
> O32 / 32-bit: Always copy 4 stack arguments." (also in 2012) which made
> the kernel unconditionally copy 4 stack arguments, even when the syscall
> doesn't even use the arguments.
>
> I guess this would be reasonably easy to fix up in musl, but let's also
> get the linux-mips people's opinions, as that commit obviously broke the
> kernel ABI...
This is kernel ABI breakage that should be fixed -- people running old
kernel versions with old musl binaries might suffer a regression when
upgrading, and perhaps more importantly the failure mode is just
really bad. But I think we can also work around it on the userspace
side in musl by pointing the stack pointer at some rodata (or even at
pc, e.g. copying $25 to $sp) before making the syscall.
Rich
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [musl] musl-libc/MIPS: detached thread exit broken since kernel commit 46e12c07b
2015-06-19 2:50 ` [musl] " Rich Felker
@ 2015-06-19 10:06 ` Ralf Baechle
2015-06-19 14:35 ` Rich Felker
0 siblings, 1 reply; 4+ messages in thread
From: Ralf Baechle @ 2015-06-19 10:06 UTC (permalink / raw
To: Rich Felker
Cc: Matthias Schiffer, musl, linux-mips, linux-kernel@vger.kernel.org
On Thu, Jun 18, 2015 at 10:50:32PM -0400, Rich Felker wrote:
> This is kernel ABI breakage that should be fixed -- people running old
> kernel versions with old musl binaries might suffer a regression when
> upgrading, and perhaps more importantly the failure mode is just
> really bad. But I think we can also work around it on the userspace
> side in musl by pointing the stack pointer at some rodata (or even at
> pc, e.g. copying $25 to $sp) before making the syscall.
Just to be on the safe side, make sure it is something that's readable. Core
might me mapped execute-only, that is not readable and that is a feature
which the affected kernels do support on suitable hardware.
Ralf
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [musl] musl-libc/MIPS: detached thread exit broken since kernel commit 46e12c07b
2015-06-19 10:06 ` Ralf Baechle
@ 2015-06-19 14:35 ` Rich Felker
0 siblings, 0 replies; 4+ messages in thread
From: Rich Felker @ 2015-06-19 14:35 UTC (permalink / raw
To: Ralf Baechle
Cc: Matthias Schiffer, musl, linux-mips, linux-kernel@vger.kernel.org
On Fri, Jun 19, 2015 at 12:06:26PM +0200, Ralf Baechle wrote:
> On Thu, Jun 18, 2015 at 10:50:32PM -0400, Rich Felker wrote:
>
> > This is kernel ABI breakage that should be fixed -- people running old
> > kernel versions with old musl binaries might suffer a regression when
> > upgrading, and perhaps more importantly the failure mode is just
> > really bad. But I think we can also work around it on the userspace
> > side in musl by pointing the stack pointer at some rodata (or even at
> > pc, e.g. copying $25 to $sp) before making the syscall.
>
> Just to be on the safe side, make sure it is something that's readable. Core
> might me mapped execute-only, that is not readable and that is a feature
> which the affected kernels do support on suitable hardware.
How would that happen? Do you have ELF files with 3 PT_LOAD segments?
Normally there are two and their permissions are r-x and rw-.
Rich
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^ permalink raw reply [flat|nested] 4+ messages in thread
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2015-06-19 2:07 musl-libc/MIPS: detached thread exit broken since kernel commit 46e12c07b Matthias Schiffer
2015-06-19 2:50 ` [musl] " Rich Felker
2015-06-19 10:06 ` Ralf Baechle
2015-06-19 14:35 ` Rich Felker
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