[Bug ports/15054] New: MIPS/Linux syscall restart convention not respected by INTERNAL_SYSCALL_NCS

[Bug ports/15054] New: MIPS/Linux syscall restart convention not respected by INTERNAL_SYSCALL_NCS

[macro@linux-mips.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=15054

             Bug #: 15054
           Summary: MIPS/Linux syscall restart convention not respected by
                    INTERNAL_SYSCALL_NCS
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ports
        AssignedTo: unassigned@sourceware.org
        ReportedBy: macro@linux-mips.org
                CC: carlos@systemhalted.org, roland@gnu.org
    Classification: Unclassified


We have an issue with the INTERNAL_SYSCALL_NCS wrapper in that it does not
respect the kernel's syscall restart convention.

That convention requires the instruction immediately preceding SYSCALL to
initialize $v0 with the syscall number.  Then if a restart triggers, $v0
will have been clobbered by the syscall interrupted, and needs to be
reinititalized.  The kernel will decrement the PC by 4 before switching
back to the user mode so that $v0 has been reloaded before SYSCALL is
executed again.  This implies the place $v0 is loaded from must be
preserved across a syscall, e.g. an immediate, static register, stack slot,
etc.

We use two wrapper macros to dispatch syscalls to the relevant pieces of
code: INTERNAL_SYSCALL and INTERNAL_SYSCALL_NCS.  They both ultimately
cause a piece of inline assembly to be emitted.  In the former case the
piece starts with an LI instruction that loads $v0 with an immediate number
of the syscall required.  A SYSCALL instruction then immediately follows.
In the latter case $v0 is arranged to have been preloaded and the piece
starts with a SYSCALL instruction.

That works if the syscall is executed the first time, because the compiler
will have arranged for $v0 to contain the correct value.  It does not in
the case of a syscall restart as the compiler-generated instruction
immediately preceding SYSCALL may not necessarily be one to load $v0 with
the value required.

The failure mode is unlikely to trigger as the INTERNAL_SYSCALL_NCS wrapper
is only used in a couple of places and then the offending syscall would
have to be restarted as well.  The symptom would usually be an intermittent
program failure and would be hard to debug.  The issue was noticed by code
inspection while making changes in this area.

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[Bug ports/15054] MIPS/Linux syscall restart convention not respected by INTERNAL_SYSCALL_NCS

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=15054

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugdal at aerifal dot cx

--- Comment #1 from Rich Felker <bugdal at aerifal dot cx> 2013-01-23 17:26:53 UTC ---
This issue only affects old kernels that are barely in the realm of
still-supported. See:

http://www.linux-mips.org/archives/linux-mips/2012-09/msg00072.html

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[Bug ports/15054] MIPS/Linux syscall restart convention not respected by INTERNAL_SYSCALL_NCS

[joseph at codesourcery dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15054

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2013-01-23 17:37:43 UTC ---
On Wed, 23 Jan 2013, bugdal at aerifal dot cx wrote:

> This issue only affects old kernels that are barely in the realm of
> still-supported. See:

The minimum kernel version currently supported by glibc is 2.6.16.  If the 
code can be simplified when the minimum kernel version moves to something 
more recent, it would indeed be good for the comments about the syscall 
restart convention to identify the relevant kernel version that removed 
the issue so it is clear when the complexity is obsolete.

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[Bug ports/15054] MIPS/Linux syscall restart convention not respected by INTERNAL_SYSCALL_NCS

[macro@linux-mips.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=15054

Maciej W. Rozycki <macro@linux-mips.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #3 from Maciej W. Rozycki <macro@linux-mips.org> 2013-02-05 15:25:00 UTC ---
I think the removal of code complication is not worth the hassle and
given how the MIPS ABI has been laid out there is normally no runtime
overhead from this arrangement, merely instruction shuffling.

I think addressing the issue of multiple stack pointer adjustments made
for o32 syscalls that make use of more than four arguments (the alloca
or FORCE_FRAME_POINTER hack) and the resulting space and time overhead
of unnecessary register juggling would be a better use of anyone's time
and resources.

Fixed thus:

commit b82ba2f011fc4628ceece07412846d0b4d50cac2
Author: Maciej W. Rozycki <macro@codesourcery.com>
Date:   Tue Feb 5 14:41:32 2013 +0000

    MIPS: Respect the legacy syscall restart convention.

    That convention requires the instruction immediately preceding SYSCALL
    to initialize $v0 with the syscall number.  Then if a restart triggers,
    $v0 will have been clobbered by the syscall interrupted, and needs to be
    reinititalized.  The kernel will decrement the PC by 4 before switching
    back to the user mode so that $v0 has been reloaded before SYSCALL is
    executed again.  This implies the place $v0 is loaded from must be
    preserved across a syscall, e.g. an immediate, static register, stack
    slot, etc.

    The restriction was lifted with Linux 2.6.36 kernel release and no
    special requirements are placed around the SYSCALL instruction anymore,
    however we still support older kernel binaries.

ChangeLog.mips:

2013-02-05  Maciej W. Rozycki  <macro@codesourcery.com>

    [BZ #15054]
    * sysdeps/unix/sysv/linux/mips/mips32/sysdep.h (MOVE32):
    New macro.
    (INTERNAL_SYSCALL_NCS): Use it.  Rewrite to respect the syscall
    restart convention.
    (INTERNAL_SYSCALL): Rewrite to respect the syscall restart
    convention.
    (internal_syscall0, internal_syscall1): Likewise.
    (internal_syscall2, internal_syscall3): Likewise.
    (internal_syscall4, internal_syscall5): Likewise.
    (internal_syscall6, internal_syscall7): Likewise.
    * sysdeps/unix/sysv/linux/mips/mips64/n32/sysdep.h (MOVE32):
    New macro.
    (INTERNAL_SYSCALL_NCS): Use it.  Rewrite to respect the syscall
    restart convention.
    (INTERNAL_SYSCALL): Rewrite to respect the syscall restart
    convention.
    (internal_syscall0, internal_syscall1): Likewise.
    (internal_syscall2, internal_syscall3): Likewise.
    (internal_syscall4, internal_syscall5): Likewise.
    (internal_syscall6): Likewise.
    * sysdeps/unix/sysv/linux/mips/mips64/n64/sysdep.h (MOVE32):
    New macro.
    (INTERNAL_SYSCALL_NCS): Use it.  Rewrite to respect the syscall
    restart convention.
    (INTERNAL_SYSCALL): Rewrite to respect the syscall restart
    convention.
    (internal_syscall0, internal_syscall1): Likewise.
    (internal_syscall2, internal_syscall3): Likewise.
    (internal_syscall4, internal_syscall5): Likewise.
    (internal_syscall6): Likewise.

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[Bug ports/15054] MIPS/Linux syscall restart convention not respected by INTERNAL_SYSCALL_NCS

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=15054

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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