On 08/03/2021 20:01, Jeff King wrote:
On Mon, Mar 08, 2021 at 06:36:14PM +0000, Andrzej Hunt via GitGitGadget wrote:
From: Andrzej Hunt <ajrhunt@xxxxxxxxxx>
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!
