Joseph S. Myers wrote:
> I'm testing glibc on MIPS64, little-endian, N32, O32 and N64
> multilibs.
> 
> Among the NPTL test failures seen are some arising from sigsuspend
> problems for N32: it blocks the wrong signals, so SIGCANCEL (SIGRTMIN)
> is blocked despite glibc's carefully excluding it from sets of signals
> to block.  Specifically, testing suggests it blocks signal N^32
> instead of signal N, so (in the example tested) blocking SIGUSR1 (17)
> blocks signal 49 instead.
> 
> glibc's sigset_t uses an array of unsigned long, as does the kernel.
> In both cases, signal N+1 is represented as
> (1UL << (N % (8 * sizeof (unsigned long)))) in word number
> (N / (8 * sizeof (unsigned long))).
> 
> Thus the N32 glibc uses an array of 32-bit words and the N64 kernel
> uses an array of 64-bit words.  For little-endian, the layout is the
> same, with signals 1-32 in the first 4 bytes, signals 33-64 in the
> second, etc.; for big-endian, userspace has that layout while in the
> kernel each 8 bytes have the two halves swapped from the userspace
> layout.
> 
> The N32 sigsuspend syscall uses sigset_from_compat to convert the
> userspace sigset to kernel format.  If __COMPAT_ENDIAN_SWAP__ is *not*
> set, this uses logic of the form
> 
>   set->sig[0] = compat->sig[0] | (((long)compat->sig[1]) << 32 )
> 
> to convert the userspace sigset to a kernel one.  This looks correct
> to me for both big and little endian, given that in userspace
> compat->sig[1] will represent signals 33-64, and so will the high 32
> bits of set->sig[0] in the kernel.  If however __COMPAT_ENDIAN_SWAP__
> *is* set, as it is for __MIPSEL__, it uses
> 
>   set->sig[0] = compat->sig[1] | (((long)compat->sig[0]) << 32 );
> 
> which seems incorrect for both big and little endian, and would
> explain the observed symptoms.
> 
> This code is the only use of __COMPAT_ENDIAN_SWAP__, so if incorrect
> then that macro serves no purpose, in which case something like the
> following patch would seem appropriate to remove it.



You might find the attached patch of interest.

We've been using it gentoo-side for awhile now, as it allowed some N32 programs 
to work correctly.  Namely, a configure test in glib would trigger a non-fatal 
oops in the kernel due to an issue this patch fixes.  Daniel Jacobwitz wrote it 
up when he apparently stumbled across the issue (something wonky in n32's 
sigsuspend, as the name indicates.  I forget the specifics, though), but I'm 
unsure if the issue is in any way connected to what you're seeing as well.


--Kumba

-- 
Gentoo/MIPS Team Lead
Gentoo Foundation Board of Trustees

"Such is oft the course of deeds that move the wheels of the world: small hands 
do them because they must, while the eyes of the great are elsewhere."  --Elrond