Hi Andreas,

one thing I usually do in such cases is to rebuild the package adding
"-fsanitize=address -O0" flags (optimization just to understand better
what happens in the source). This switches the address sanitizer on
<https://github.com/google/sanitizers/wiki/AddressSanitizer>. This can
test if a local variable is accidently overwritten (by an off-by-one
error or similar). Often it finds many more bugs which one can turn
upstream into bonus points...

Otherwise I see no other chance than to go through the debugger and see
where the strange address was set. 0x7 however sounds that somewhere a
small integer was assigned to the pointer, so I would try the sanitizing
stuff first.



Andreas Tille <andr...@an3as.eu> writes:
> Hi,
> as reported in bug #907624 ffindex autopkgtest fails with SIGSEGV in sid
> and buster.  I've tested in stretch (gcc 6.3) and the code works fine.
> I've reported upstream[1] the results of my gdb session where I was able
> to find the exact code line[2] where the SIGSEGV is thrown.  It turns out
> that the elements of a structure are not accessible:
>    (gdb) print entry->offset
>    Cannot access memory at address 0x7
> (full gdb log under [1] or in the bug log).
> In fact I tried in some more detailed debugging that any attempt to
> access one of the structure elements even for instance only injecting
> something like 
>    if ( !entry->offset ) {
> in line 554 will trigger the SIGSEGV.  The values of the structure are
> set in line 350[3] and are OK there.  The funktion that contains the
> failing line is action() [4] and called via a pointer to this function
> in line 563[5] (I admit I have no real idea why this pointer to a
> function should be needed.  Its the only function that is used in this
> place and IMHO only adds an extra layer of complexity.)
> The structure is declared in the header file[6].
> I admit I fail to see why the code works under stretch with gcc 6.3
> but fails with gcc 8.2.
> Any idea?
> Kind regards
>        Andreas.
> [1] https://github.com/soedinglab/ffindex_soedinglab/issues/7
> [2] https://salsa.debian.org/med-team/ffindex/blob/master/src/ffindex.c#L554
> [3] https://salsa.debian.org/med-team/ffindex/blob/master/src/ffindex.c#L350
> [4] https://salsa.debian.org/med-team/ffindex/blob/master/src/ffindex.c#L541
> [5] https://salsa.debian.org/med-team/ffindex/blob/master/src/ffindex.c#L563
> [6] https://salsa.debian.org/med-team/ffindex/blob/master/src/ffindex.h#L30

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