On Mon, Oct 25, 2021 at 07:46:11AM +0000, David Laight wrote:
From: Willy Tarreau
Sent: 24 October 2021 18:28
After re-checking in the spec and comparing stack offsets with glibc, The last pushed argument must be 16-byte aligned (i.e. aligned before the call) so that in the callee esp+4 is multiple of 16, so the principle is the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that 32-bit code using SSE2 or MMX could have been affected. In addition the frame pointer ought to be zero at the deepest level.
...
/* startup code */ +/*
- i386 System V ABI mandates:
- last pushed argument must be 16-byte aligned.
- The deepest stack frame should be set to zero
I'm pretty sure that the historic SYSV i386 ABI only every required 4-byte alignment for the stack.
At some point it got 'randomly' changed to 16-byte. I don't think this happened until after compiler support for SSE2 intrinsics was added.
It's very possible because I've done a number of tests and noticed that in some cases the called functions' stack doesn't seem to be more than 4-aligned. However the deepest function in the stack starts with an aligned stack so I prefer to follow this same rule.
Willy