Andi Kleen wrote:
On Wed, Sep 14, 2005 at 07:55:53AM -0700, Linus Torvalds wrote:
On Wed, 14 Sep 2005, Daniel Jacobowitz wrote:
The comment for the relevant bits of the GCC configuration says it won't
assume this for x86, but I believe that comment is out of date. I think
it'll assume 16-byte alignment on entrance to non-main() functions.
Well, that's kind of the point. We _do_ have the stack aligned on
entrance, but it looks like gcc wants it _non-aligned_. It seems to want
it offset by the "return address push" - ie it seems to expect that it was
aligned before the "call", but entry into the next function will thus
_never_ be aligned.
So the kernel actually seems to have it _too_ aligned right now.
Yes it's wrong. I would recommend to apply Markus' patch for i386
and x86-64.
-Andi
Here is a somewhat simplified version of my previous patch with
updated comments.
Attached is also a new small user-space test program which does not
depend on any special gcc features and should trigger the problem on all
machines.
~Markus
P.S. I have not been involved in lkml back since 1999, so I currently don't
know whom to bug to get this patch applied, esp. as there seems to be no
official i386 maintainer.
--
Markus Oberhumer, <[email protected]>, http://www.oberhumer.com/
[PATCH] i386: fix stack alignment for signal handlers
This patches fixes the setup of the alignment of the signal frame, so
that all signal handlers are run with a properly aligned stack frame.
The current code "over-aligns" the stack pointer so that the stack
frame is effectively always mis-aligned by 4 bytes. But what we really
want is that on function entry ((sp + 4) & 15) == 0.
Signed-off-by: Markus F.X.J. Oberhumer <[email protected]>
arch/i386/kernel/signal.c | 6 +++++-
arch/x86_64/ia32/ia32_signal.c | 6 +++++-
2 files changed, 10 insertions(+), 2 deletions(-)
Index: linux-2.6.git/arch/i386/kernel/signal.c
===================================================================
--- linux-2.6.git.orig/arch/i386/kernel/signal.c
+++ linux-2.6.git/arch/i386/kernel/signal.c
@@ -338,7 +338,11 @@
esp = (unsigned long) ka->sa.sa_restorer;
}
- return (void __user *)((esp - frame_size) & -8ul);
+ esp -= frame_size;
+ /* Align the stack pointer according to the i386 ABI,
+ * i.e. so that on function entry ((sp + 4) & 15) == 0. */
+ esp = ((esp + 4) & -16ul) - 4;
+ return (void __user *) esp;
}
/* These symbols are defined with the addresses in the vsyscall page.
Index: linux-2.6.git/arch/x86_64/ia32/ia32_signal.c
===================================================================
--- linux-2.6.git.orig/arch/x86_64/ia32/ia32_signal.c
+++ linux-2.6.git/arch/x86_64/ia32/ia32_signal.c
@@ -425,7 +425,11 @@
rsp = (unsigned long) ka->sa.sa_restorer;
}
- return (void __user *)((rsp - frame_size) & -8UL);
+ rsp -= frame_size;
+ /* Align the stack pointer according to the i386 ABI,
+ * i.e. so that on function entry ((sp + 4) & 15) == 0. */
+ rsp = ((rsp + 4) & -16ul) - 4;
+ return (void __user *) rsp;
}
int ia32_setup_frame(int sig, struct k_sigaction *ka,
/* test signal stack alignment (sigframe)
*
* a small user-space demo program to show that the signal stack
* is currently mis-aligned on i386-linux
*
* Markus F.X.J. Oberhumer <[email protected]>
*/
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
void sighandler(int);
void test(void);
volatile unsigned long sp = 0;
/* assembler prologue code that stores the stack pointer into 'sp'
* and then jumps to the real function */
#if defined(__i386__)
asm(
".text\n"
"sighandler:\n"
"lea 4(%esp), %eax\n"
"mov %eax, (sp)\n"
"jmp do_sighandler\n"
"test:\n"
"lea 4(%esp), %eax\n"
"mov %eax, (sp)\n"
"jmp do_test\n"
".globl main\n"
"main:\n"
"lea 4(%esp), %eax\n"
"mov %eax, (sp)\n"
"jmp do_main\n"
);
#elif defined(__x86_64__)
asm(
".text\n"
"sighandler:\n"
"lea 8(%rsp), %rax\n"
"mov %rax, sp(%rip)\n"
"jmp do_sighandler\n"
"test:\n"
"lea 8(%rsp), %rax\n"
"mov %rax, sp(%rip)\n"
"jmp do_test\n"
".globl main\n"
"main:\n"
"lea 8(%rsp), %rax\n"
"mov %rax, sp(%rip)\n"
"jmp do_main\n"
);
#else
#error "arch not supported - please insert your code here"
#endif
void do_sighandler(void)
{
printf("in sighandler: sp = 0x%lx\n", sp);
}
void do_test(void)
{
printf("in test : sp = 0x%lx\n", sp);
signal(SIGUSR1, sighandler);
raise(SIGUSR1);
}
int do_main(void)
{
printf("in main : sp = 0x%lx\n", sp);
test();
printf("All done.\n");
return 0;
}
/* vim:set ts=4 et: */
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