Re: [PATCH 1/7] symbolic-ref: don't leak shortened refname in check_symref()

On 08/03/2021 20:01, Jeff King wrote:

On Mon, Mar 08, 2021 at 06:36:14PM +0000, Andrzej Hunt via GitGitGadget wrote:

quoted

From: Andrzej Hunt <redacted>

This leak has existed since:
9ab55daa55 (git symbolic-ref --delete $symref, 2012-10-21)

This leak was found when running t0001 with LSAN, see also LSAN output
below:

Direct leak of 19 byte(s) in 1 object(s) allocated from:
     #0 0x486514 in strdup /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/asan/asan_interceptors.cpp:452:3
     #1 0x9ab048 in xstrdup /home/ahunt/oss-fuzz/git/wrapper.c:29:14
     #2 0x8b452f in refs_shorten_unambiguous_ref /home/ahunt/oss-fuzz/git/refs.c
     #3 0x8b47e8 in shorten_unambiguous_ref /home/ahunt/oss-fuzz/git/refs.c:1287:9
     #4 0x679fce in check_symref /home/ahunt/oss-fuzz/git/builtin/symbolic-ref.c:28:14
     #5 0x679ad8 in cmd_symbolic_ref /home/ahunt/oss-fuzz/git/builtin/symbolic-ref.c:70:9
     #6 0x4cd60d in run_builtin /home/ahunt/oss-fuzz/git/git.c:453:11
     #7 0x4cb2da in handle_builtin /home/ahunt/oss-fuzz/git/git.c:704:3
     #8 0x4ccc37 in run_argv /home/ahunt/oss-fuzz/git/git.c:771:4
     #9 0x4cac29 in cmd_main /home/ahunt/oss-fuzz/git/git.c:902:19
     #10 0x69cc6e in main /home/ahunt/oss-fuzz/git/common-main.c:52:11
     #11 0x7f98388a4349 in __libc_start_main (/lib64/libc.so.6+0x24349)

As a general template for fixing leaks, this information seems pretty
good. You might want to give a brief reason for why it's a leak (like
you do already in the second patch). Here it just would be something
like:

   shorten_unambiguous_ref() returns an allocated string. We have to
   track it separately from the const refname.

Or whatever. It doesn't need to be a novel, but just give an overview of
what's going that makes the diff obvious.

Good point - I've copied this one verbatim - but it's also a good thing to remember if/when I fix further leaks!

There's also an idiom in Git's codebase when a const pointer may alias
unowned storage, or a buffer that needs to be freed. Something like:

   if (print) {
           char *to_free = NULL;
	  if (shorten)
	          refname = to_free = shorten_unambiguous_ref(refname, 0);
	  puts(refname);
	  free(to_free);
   }

That avoids duplicating the part of the code that handles the variable.
In this case it is only a single line, but IMHO it's still easier to
read, as it makes clear that we call puts() in either case.

That's a nice pattern, and will probably be useful for future leak fixes too - I've made this change too!