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N32 sigset and __COMPAT_ENDIAN_SWAP__

To: linux-mips@linux-mips.org
Subject: N32 sigset and __COMPAT_ENDIAN_SWAP__
From: "Joseph S. Myers" <joseph@codesourcery.com>
Date: Thu, 8 Jun 2006 01:36:29 +0000 (UTC)
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
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.


Signed-off-by: Joseph Myers <joseph@codesourcery.com>

---

diff -rupN linux-2.6.17-rc6.orig/include/asm-mips/compat.h 
linux-2.6.17-rc6/include/asm-mips/compat.h
--- linux-2.6.17-rc6.orig/include/asm-mips/compat.h     2006-06-08 
01:05:13.000000000 +0000
+++ linux-2.6.17-rc6/include/asm-mips/compat.h  2006-06-08 01:04:10.000000000 
+0000
@@ -145,8 +145,5 @@ static inline void __user *compat_alloc_
 
        return (void __user *) (regs->regs[29] - len);
 }
-#if defined (__MIPSEL__)
-#define __COMPAT_ENDIAN_SWAP__         1
-#endif
 
 #endif /* _ASM_COMPAT_H */
diff -rupN linux-2.6.17-rc6.orig/kernel/compat.c 
linux-2.6.17-rc6/kernel/compat.c
--- linux-2.6.17-rc6.orig/kernel/compat.c       2006-06-08 01:05:13.000000000 
+0000
+++ linux-2.6.17-rc6/kernel/compat.c    2006-06-08 01:03:47.000000000 +0000
@@ -729,17 +729,10 @@ void
 sigset_from_compat (sigset_t *set, compat_sigset_t *compat)
 {
        switch (_NSIG_WORDS) {
-#if defined (__COMPAT_ENDIAN_SWAP__)
-       case 4: set->sig[3] = compat->sig[7] | (((long)compat->sig[6]) << 32 );
-       case 3: set->sig[2] = compat->sig[5] | (((long)compat->sig[4]) << 32 );
-       case 2: set->sig[1] = compat->sig[3] | (((long)compat->sig[2]) << 32 );
-       case 1: set->sig[0] = compat->sig[1] | (((long)compat->sig[0]) << 32 );
-#else
        case 4: set->sig[3] = compat->sig[6] | (((long)compat->sig[7]) << 32 );
        case 3: set->sig[2] = compat->sig[4] | (((long)compat->sig[5]) << 32 );
        case 2: set->sig[1] = compat->sig[2] | (((long)compat->sig[3]) << 32 );
        case 1: set->sig[0] = compat->sig[0] | (((long)compat->sig[1]) << 32 );
-#endif
        }
 }
 


-- 
Joseph S. Myers
joseph@codesourcery.com

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