Re: dynlink.c: bug in reclaim_gaps leading to segfault in __libc_exit_fini

[Timo Teras <timo.teras@iki.fi>]

On Tue, 16 Feb 2016 22:55:50 +0100
Szabolcs Nagy <nsz@port70.net> wrote:

> * Hugues Bruant <hugues@aerofs.com> [2016-02-16 16:30:42 -0500]:
> > Affects both 1.1.12 and 1.1.13
> > 
> > Tracked down with valgrind in Alpine Linux 3.3.
> > 
> > The dmg tool build from https://github.com/aerofs/libdmg-hfsplus
> > links to a handful shared libs. The following message is seen
> > immediately at start:
> > 
> > ==59== Invalid free() / delete / delete[] / realloc()
> > ==59==    at 0x4C92B0E: free (vg_replace_malloc.c:530)
> > ==59==    by 0x4056F68: reclaim_gaps (dynlink.c:488)
> > ==59==    by 0x405743D: map_library (dynlink.c:708)
> > ==59==    by 0x4057EF3: load_library (dynlink.c:1014)
> > ==59==    by 0x4058CA8: load_preload (dynlink.c:1112)
> > ==59==    by 0x4058CA8: __dls3 (dynlink.c:1581)
> > ==59==    by 0x405856A: __dls2 (dynlink.c:1383)
> > ==59==    by 0x405655E: ??? (in /lib/ld-musl-x86_64.so.1)
> > ==59==    by 0x3: ???
> > ==59==    by 0xFFF000E3A: ???
> > ==59==    by 0xFFF000E3E: ???
> > ==59==    by 0xFFF000E44: ???
> > ==59==    by 0xFFF000E86: ???
> > 
> > Afterwards, the program proceeds with no issue, until it exists, at
> > which point a segfault is triggered when cleaning up shared
> > libraries: 
> 
> this is not a bug.

It is compliance issue. POSIX says about free:
--
The free() function shall cause the space pointed to by ptr to be
deallocated; that is, made available for further allocation. If ptr is
a null pointer, no action shall occur. Otherwise, if the argument does
not match a pointer earlier returned by a function in POSIX.1-2008 that
allocates memory as if by malloc(), or if the space has been
deallocated by a call to free() or realloc(), the behavior is undefined.
--

While overloading allocators are not supported, they'd break at this
too. And it'll be highly annoying if someone decides to test a new
memory allocator inside musl and does not know about this one exception.

> valgrind is not aware of dynamic linker internals,
> you have to use a musl specific suppression file
> to hide this message (but i dont know if anybody
> wrote such thing for valgrind).

Well - musl really should introduce __donatemem or similar for this
purpose, and not overload the standard free() function. This would make
the valgrind warning go away.

I'd rather not write a suppression for the above, since the internals
are misusing/overloading a standard api call against posix.

Technically valgrind is detecting a valid case for misuse of free().
While in context of standard musl allocator it's ok.

Thanks,
Timo