Messages in this thread | | | Date | Tue, 14 Oct 2003 11:45:34 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: [x86] Access off the bottom of stack causes a segfault? |
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On Tue, 14 Oct 2003, Chris Lattner wrote:
> > > Generated code: > > > .intel_syntax > > > ... > > > main: > > > mov DWORD PTR [%ESP - 16004], %EBP # Save EBP to stack > > ^^^^^^^^^^^^ > > > > Yes, this is the problem (even Windows does that IIRC). > > Ok, I realize what's going on here. The question is, why does the linux > kernel consider this to be a bug? Where (in the X86 specs) is it > documented that it's illegal to access off the bottom of the stack? > > My compiler does a nice leaf function optimization where it does not even > bother to adjust the stack for leaf functions, which eliminates the adds > and subtracts entirely from these (common) functions. This completely > invalidates the optimization. > > Since I'm going to have to live with lots of preexisting kernels, it looks > like I'm going to have to disable it entirely, which is disappointing. > I'm still curious though why this is thought to be illegal.
I don't know, I didn't code it :) I discovered it a few months ago while I was doing some portable code to measure the maximum stack growth of our software. I was doing something like:
memset(%esp - SAFE, 'S', SIZE);
and it was puking on me on both Linux and Windows. An alloca+memset solved the problem in both environments.
- Davide
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