[Bug glob/20707] New: gl_pathv entries not set to NULL with GLOB_DOOFFS

["mike at php dot net" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20707

            Bug ID: 20707
           Summary: gl_pathv entries not set to NULL with GLOB_DOOFFS
           Product: glibc
           Version: 2.24
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: glob
          Assignee: unassigned at sourceware dot org
          Reporter: mike at php dot net
  Target Milestone: ---

Created attachment 9568
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9568&action=edit
test program

When calling `glob(pattern, GLOB_DOOFFS, NULL, &gl)` where `gl.gl_offs>0`,
program flow does not ensure that `gl.gl_pathv[0..gl_offs]` entries are
actually set to NULL, while POSIX as well as Linux man pages say that the
reserved slots contain NULL pointers.

Allocation of gl_pathv does actually happen at [1] with an `realloc(NULL)` not
at [2] where initialization of `gl_pathv[0..gl_offs]` happens.

I made this observation by accident, noticing that a random count of reserved
elements of `gl_pathv` point to random addresses.

The attached test program crashes using address sanitizer, to be built with
`-fsanitize=address -lasan`.

Thanks for your time.

[1]
https://sourceware.org/git/?p=glibc.git;a=blob;f=posix/glob.c;h=ea4b0b61eb17fd38a7dc379e018e1b92abdf29a9;hb=fdfc9260b61d3d72541f18104d24c7bcb0ce5ca2#l1007
[2]
https://sourceware.org/git/?p=glibc.git;a=blob;f=posix/glob.c;h=ea4b0b61eb17fd38a7dc379e018e1b92abdf29a9;hb=fdfc9260b61d3d72541f18104d24c7bcb0ce5ca2#l478

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