Re: i386 ABI and the stack

On Fri, 2006-06-23 at 22:43 -0400, Albert Cahalan wrote:
On 6/23/06, Linus Torvalds <torvalds@xxxxxxxx> wrote:
On Fri, 23 Jun 2006, H. Peter Anvin wrote:

The x86-64 ABI has a 128-byte(*) zone that is safe from signals etc, so you
can use a small amount of stack below the stackpointer safely. Not so on
x86.

Adding a small redzone like this to i386 would be easy, though -- just drop
the stack pointer by that much when creating a signal frame. 128 bytes isn't
enough to interfere with libraries.

However, any binaries created with that in mind would be
buggy-by-definition on older kernels, so I don't think it's worth it.

Since gcc-2.96 would access 256 bytes below the stack pointer
(according to the valgrind man page), the kernel needs to allow
for this in signal handlers anyway.

only a handful buggy editions of that compiler did in a few corner
cases. And they were really buggy, and they were corner cases. Nobody
should be using a compiler like that; and nobody is expected to compile
software with a broken version of that compiler (iirc the window in
which it was broken was really small). There is a limit to userspace
brokenness that the kernel should work around. This imo is on the other
side of the line.

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