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[PATCH] i386 TLS_INIT_TP might produce bogus asm changing stack pointer.
- From: Mark Wielaard <mjw at redhat dot com>
- To: libc-alpha at sourceware dot org
- Cc: Siddhesh Poyarekar <siddhesh at redhat dot com>, Florian Weimer <fweimer at redhat dot com>, Mark Wielaard <mjw at redhat dot com>
- Date: Tue, 26 Aug 2014 10:27:45 +0200
- Subject: [PATCH] i386 TLS_INIT_TP might produce bogus asm changing stack pointer.
- Authentication-results: sourceware.org; auth=none
TLS_INIT_TP in sysdeps/i386/nptl/tls.h uses some hand written asm to
generate a set_thread_area that might result in exchanging ebx and esp
around the syscall causing introspection tools like valgrind to loose
track of the user stack. Just use INTERNAL_SYSCALL which makes sure
esp isn't changed arbitrarily.
Before the patch the code would generate:
mov $0xf3,%eax
movl $0xfffff,0x8(%esp)
movl $0x51,0xc(%esp)
xchg %esp,%ebx
int $0x80
xchg %esp,%ebx
Using INTERNAL_SYSCALL instead will generate:
movl $0xfffff,0x8(%esp)
movl $0x51,0xc(%esp)
xchg %ecx,%ebx
mov $0xf3,%eax
int $0x80
xchg %ecx,%ebx
Thanks to Florian Weimer for analysing why the original code generated
the bogus esp usage:
_segdescr.desc happens to be at the top of the stack, so its address
is in %esp. The asm statement says that %3 is an input, so its value
will not change, and GCC can use %esp as the input register for the
expression &_segdescr.desc. But the constraints do not fully describe
the asm statement because the %3 register is actually modified, albeit
only temporarily.
https://bugzilla.redhat.com/show_bug.cgi?id=1133134
* sysdeps/i386/nptl/tls.h (TLS_INIT_TP): Use INTERNAL_SYSCALL
to call set_thread_area instead of hand written asm.
---
ChangeLog | 5 +++++
sysdeps/i386/nptl/tls.h | 8 ++------
2 files changed, 7 insertions(+), 6 deletions(-)
diff --git a/sysdeps/i386/nptl/tls.h b/sysdeps/i386/nptl/tls.h
index ac9c9a2..052ea64 100644
--- a/sysdeps/i386/nptl/tls.h
+++ b/sysdeps/i386/nptl/tls.h
@@ -231,12 +231,8 @@ tls_fill_user_desc (union user_desc_init *desc,
tls_fill_user_desc (&_segdescr, -1, _thrdescr); \
\
/* Install the TLS. */ \
- asm volatile (TLS_LOAD_EBX \
- "int $0x80\n\t" \
- TLS_LOAD_EBX \
- : "=a" (_result), "=m" (_segdescr.desc.entry_number) \
- : "0" (__NR_set_thread_area), \
- TLS_EBX_ARG (&_segdescr.desc), "m" (_segdescr.desc)); \
+ INTERNAL_SYSCALL_DECL (err); \
+ _result = INTERNAL_SYSCALL (set_thread_area, err, 1, &_segdescr.desc); \
\
if (_result == 0) \
/* We know the index in the GDT, now load the segment register. \
--
1.9.3