[Bug libc/18292] Invalid pointer dereference in nsswitch.c:nss_new_service()

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18292

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |fweimer at redhat dot com
              Flags|                            |security-

--- Comment #10 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Justin N. Ferguson from comment #1)
> repro:
> set prev_size member of chunk to non-zero value that doesn't properly
> dereference. execute "ls -al".

I assume that this is about the prev_size member of the following chunk.  Have
you increased the chunk size accordingly (by the size of a pointer)?  If not,
your malloc invalid because prev_size is only available if the preceding chunk
is free, as explained in comment #2.

The fundamental idea behind the free space management in dlmalloc and similar
allocators is to use the trailing end of a free chunk to store a pointer (or
offset) to its beginning, so that when the subsequent chunk is freed, it can be
merged with the preceding one.  Without the information prev_size provides, it
would not be possible to find the start of the chunk to merge with.  This is
explained on page 28 of <ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps> and
sometimes called a âboundary tagâ mechanism.

valgrind also does not show any out-of-bounds references, and it is usually
very precise for such issues.

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