[Sheflug] Re: [Sheflug - Sheffield Linux User's Group] Re: Segfaults & ld.sowierditude

["Stephen J. Turnbull" <nospam [at] sk.tsukuba.ac.jp>]

>>>>> "Bob" == Bob Ham <u9rah [at] dcs.shef.ac.uk> writes:

    Bob> Like I say, it's not my instruction that's segfaulting.

A bogus pointer can point _anywhere_.  The code manipulating the bogus
pointer doesn't care; the code that gets mashed, does.  Your (bogus)
code doesn't crash, it's the code that lives in memory that gets
smashed by the bogus code crashes.  Or code that uses data that gets
smashed.  Many versions of malloc keep malloc housekeeping data mixed
with the heap:

+--------------+
| malloc parms |
+--------------+
|              | <-- aPtr
|  heap data   |
|              |
+--------------+

Guess what happens to malloc if you do aPtr++ = rand(); ?

(I don't know if this is true of dlmalloc.)

    >> Have you tried Electric Fence and cousins?

    Bob> Never heard of it; care to expand on that?

Standard package in all the distributions AFAIK (Debian and
TurboLinux, anyway).  Replaces malloc and automatically watches all
pointers amlloced for evil behavior.


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