Re: [tip:x86/asm] x86/asm/entry/64: Remove unused thread_struct::usersp

[Denys Vlasenko <dvlasenk@redhat.com>]

On 03/16/2015 05:47 PM, Borislav Petkov wrote:
> On Mon, Mar 16, 2015 at 05:05:53AM -0700, tip-bot for Denys Vlasenko wrote:
>> Commit-ID:  245214a155c711764b3853189441c9f8aeb058b3
>> Gitweb:     http://git.kernel.org/tip/245214a155c711764b3853189441c9f8aeb058b3
>> Author:     Denys Vlasenko <dvlasenk@redhat.com>
>> AuthorDate: Tue, 10 Mar 2015 11:45:07 +0100
>> Committer:  Ingo Molnar <mingo@kernel.org>
>> CommitDate: Tue, 10 Mar 2015 13:56:11 +0100
>>
>> x86/asm/entry/64: Remove unused thread_struct::usersp
>>
>> All manipulations of PER_CPU(old_rsp) in C code are removed:
>> it is not used on SYSRET return, so storing anything there is
>> pointless.
>>
>> This also allows us to get rid of thread_struct::usersp,
>> which was needed only to set PER_CPU(old_rsp) for correct
>> return from fork/clone.
>>
>> Tweak a few comments as well: we no longer have "partial stack frame",
>> ever.
>>
>> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: Alexei Starovoitov <ast@plumgrid.com>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Cc: H. Peter Anvin <hpa@zytor.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Oleg Nesterov <oleg@redhat.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Will Drewry <wad@chromium.org>
>> Link: http://lkml.kernel.org/r/1425984307-2143-2-git-send-email-dvlasenk@redhat.com
>> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> 
> So this patch is causing all kinds of segfaults when booting my kvm
> guest here, see below.

I built defconfig kernel from tip, and tested it again under qemu-kvm.
Works for me with and without this change.

What's your config? What distro do you run in the guest?

> Reverting it makes the segfaults go away but from looking at the patch,
> I have no idea why it would even cause those segfaults.

Yep. This is one of those cases where "it must be completely safe"...

> [    5.285547] kmod[1316]: segfault at 738c08 ip 0000000000738c08 sp 00007ffdb6079c68 error 15
> [    9.537606] tput[2716]: segfault at 0 ip           (null) sp 00007fffffffdbd0 error 14 in tput[400000+3000]
> 					  ^^^^^^^^^^^^^^^^^
> 
> Looks like rIP has went off somewhere in the weeds.
> Hmmm...
> 
> [    4.593374] grep[998]: segfault at 7ffc3a9f4378 ip 00007fb8409fe1df sp 00007ffc3a9f4378 error 4 in ld-2.13.so[7fb8409e8000+20000]
> [    4.593374] grep[998]: segfault at 7ffc3a9f4378 ip 00007fb8409fe1df sp 00007ffc3a9f4378 error 4 in ld-2.13.so[7fb8409e8000+20000]
> 
> [    7.160423] sed[1999]: segfault at 7ffe9998f778 ip 00007f37deef0b52 sp 00007ffe9998f778 error 4 in libc-2.13.so[7f37dee18000+182000]
> 
> [    4.593374] grep[998]: segfault at 7ffc3a9f4378 ip 00007fb8409fe1df sp 00007ffc3a9f4378 error 4 in ld-2.13.so[7fb8409e8000+20000]
> [    7.160423] sed[1999]: segfault at 7ffe9998f778 ip 00007f37deef0b52 sp 00007ffe9998f778 error 4 in libc-2.13.so[7f37dee18000+182000]
> 
> [    4.593374] grep[998]: segfault at 7ffc3a9f4378 ip 00007fb8409fe1df sp 00007ffc3a9f4378 error 4 in ld-2.13.so[7fb8409e8000+20000]
> [    7.160423] sed[1999]: segfault at 7ffe9998f778 ip 00007f37deef0b52 sp 00007ffe9998f778 error 4 in libc-2.13.so[7f37dee18000+182000]
> [    5.607611] sed[1350]: segfault at 7ffddd4a4bf0 ip 00007ff24a11fafc sp 00007ffddd4a4bf0 error 4 in libc-2.13.so[7ff24a050000+182000]

This does not look entirely random.
Can you take a look what's at those locations in ld-2.13.so and libc-2.13.so?