[Bug nptl/18726] Futexes are broken on MIPS/n32

["joseph at codesourcery dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18726

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Mon, 27 Jul 2015, oss at malat dot biz wrote:

> Hi,
> there is a problem in generic syscall implementation, which breaks futexes,
> which break pthread_* functions using them.

What glibc and Linux kernel versions did you test with?

> The problem is in
>   sysdeps/unix/sysv/linux/mips/mips64/n32/sysdep.h
> file in the macro
>   #define ARGIFY(X) ((long long) (__typeof__ ((X) - (X))) (X))
> which is then used to load syscall arguments into registers. I assume its
> purpose is to avoid sign extension of an argument if it's of unsigned type,

No, its purpose is to avoid warnings for converting 32-bit pointers to 
64-bit integers - which means that 32-bit pointers have to be converted to 
32-bit integers before being converted to 64-bit integers.  See 
<https://sourceware.org/ml/libc-ports/2007-06/msg00003.html>.

> which seems to be wrong. The CPU always does sign extensions while it's loading
> 32bit integers and this is expected by Linux and GCC. The problem arises, if an

Linux (versions since SYSCALL_WRAPPERS, which I think includes all 2.6.29 
and later versions) should always do that sign extension on syscall entry 
to avoid a security issue (CVE-2009-0029, see bug 4459).  The minimum 
kernel supported by current glibc is 2.6.32 (likely to move to 3.2 in the 
not too distant future when the 2.6.32 kernel series ceases to be 
maintained).

> This can be fixed by replacing ARGIFY with a simple cast to long, however it
> wouldn't work for a syscall, which takes 64bit argument, but I do not know
> about a syscall, which would take such an argument on 32bit platform (e.g.

n32 syscalls take arguments in the same way as n32 userspace functions, in 
general (that is, 64-bit arguments go in single registers).  Look at the 
various *.c files in sysdeps/unix/sysv/linux/mips/mips64/n32/ for code 
using the syscall macros in such a way that requires them to work with 
64-bit integer arguments.

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