Re: Detecting illegal memory access

[Jan Engelhardt <jengelh@linux01.gwdg.de>]

>
>A user program can trap EFAULT.

There are no syscalls involved. It is a [arch-dependent] stack backtrace 
that walks up EBP. But object files compiled with -fomit-frame-pointer have 
random values in %ebp. So when doing

    ebp = *(void **)ebp;

it either succeeds or throws SIGSEGV. Catching signals is catchy because 
the backtrace might be done in a threaded application. Does *jmp() work 
with threads?


>However, you need to use siglongjmp()
>to get back into your program and continue, plus you need to do very
>careful coding to mask and unmask the appropriate signals though it
>all. Since protection is on a per-page basis, one could make a program
>that attempts to R/W from virtual page 0 on up, recoding success or
>failure for each page.


Jan Engelhardt
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