Messages in this thread | | | From | Keith Owens <> | Subject | Unbalanced stack usage in arch/i386/math-emu/wm_sqrt.S | Date | Mon, 13 Aug 2007 17:06:16 +1000 |
| |
Originally sent to the maintainer of the i386 math-emu code (billm@suburbia.net) but that mail was bounced[1]. Is anybody maintaining the math-emu code and do we even care about it anymore?
I am doing static code analysis on the kernel and have found a stack imbalance in arch/i386/math-emu/wm_sqrt.S. 2.6.23-rc2, but it has probably been there for a while.
The code starts off with
pushl %ebp movl %esp,%ebp #ifndef NON_REENTRANT_FPU subl $28,%esp #endif /* NON_REENTRANT_FPU */ pushl %esi pushl %edi pushl %ebx
At this point, the code is using 0x2c bytes of stack space.
.... do some work
sqrt_stage_2_finish: sarl $1,%ecx /* divide by 2 */ rcrl $1,%eax
/* Form the new estimate in %esi:%edi */ movl %eax,%edi addl %ecx,%esi
jnz sqrt_stage_2_done /* result should be [1..2) */
... still using 0x2c bytes of stack space
#ifdef PARANOID /* It should be possible to get here only if the arg is ffff....ffff */ cmp $0xffffffff,FPU_fsqrt_arg_1 jnz sqrt_stage_2_error #endif /* PARANOID */
/* The best rounded result. */ xorl %eax,%eax decl %eax movl %eax,%edi movl %eax,%esi movl $0x7fffffff,%eax jmp sqrt_round_result
#ifdef PARANOID sqrt_stage_2_error:
.... 0x2c bytes of stack space
pushl EX_INTERNAL|0x213
.... 0x30 bytes of stack space
call EXCEPTION
.... EXCEPTION is FPU_exception which only aborts if __DEBUG__ is defined, __DEBUG__ is not defined. So FPU_exception will return and we still have 0x30 bytes of stack used. But the code drops through to sqrt_stage_2_done which (like the rest of the code) only expects 0x2c bytes of stack ===> stack imbalance.
#endif /* PARANOID */
sqrt_stage_2_done:
The obvious fix is to add 'pop %eax' after 'call EXCEPTION' which will remove the extra word from the stack. Alas that only fixes the stack imbalance, but does it even make sense for the code to continue after calling EXCEPTION?
[1] <billm@suburbia.net>: host suburbia.com.au[203.24.247.1] said: 554 <billm@suburbia.net>: Recipient address rejected: Access denied (in reply to RCPT TO command)
The URL listed in MAINTAINERS for FPU Emulator gets a 404 as well.
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |