On 22/06/2021 11:02, Phillip Wood wrote:

Hi Elijah

On 21/06/2021 22:39, Elijah Newren wrote:

quoted

On Sun, Jun 20, 2021 at 11:29 AM Phillip Wood 
[off-list ref] wrote:

quoted

Hi Andrzej

Thanks for working on removing memory leaks from git.

On 20/06/2021 16:12, andrzej@ahunt.org wrote:

quoted

From: Andrzej Hunt <redacted>

This change:
- xstrdup()'s all string being used for replace_opts.strategy, to

I think you mean replay_opts rather than replace_opts.

quoted

    guarantee that replace_opts owns these strings. This is needed 
because
    sequencer_remove_state() will free replace_opts.strategy, and it's
    usually called as part of the usage of replace_opts.
- Removes xstrdup()'s being used to populate options.strategy in
    cmd_rebase(), which avoids leaking options.strategy, even in the
    case where strategy is never moved/copied into replace_opts.

quoted

These changes are needed because:
- We would always create a new string for options.strategy if we either
    get a strategy via options (OPT_STRING(...strategy...), or via
    GIT_TEST_MERGE_ALGORITHM.
- But only sometimes is this string copied into replace_opts - in which
    case it did get free()'d in sequencer_remove_state().
- The rest of the time, the newly allocated string would remain unused,
    causing a leak. But we can't just add a free because that can 
result
    in a double-free in those cases where replace_opts was populated.

An alternative approach would be to set options.strategy to NULL when
moving the pointer to replace_opts.strategy, combined with always
free()'ing options.strategy, but that seems like a more
complicated and wasteful approach.

read_basic_state() contains
         if (file_exists(state_dir_path("strategy", opts))) {
                 strbuf_reset(&buf);
                 if (!read_oneliner(&buf, state_dir_path("strategy", 
opts),
                                    READ_ONELINER_WARN_MISSING))
                         return -1;
                 free(opts->strategy);
                 opts->strategy = xstrdup(buf.buf);
         }

So we do try to free opts->strategy when reading the state from disc and
we allocate a new string. I suspect that opts->strategy is actually NULL
in when this function is called but I haven't checked. 

Thank you for noticing this. I think you're right - running an ASAN 
build past the whole test suite also didn't catch any double-frees which 
mostly confirms that opts->strategy is indeed always NULL here. But 
that's not a good reason for taking the risk.

quoted

quoted

Given that we are
allocating a copy above I think maybe your alternative approach of
always freeing opts->strategy would be better.

I will go down this route for V2. Although on further thought: instead 
of my original idea of moving the string to replay_opts (and NULL'ing 
out rebase_options->strategy), I think it's better to create a new copy 
when populating replay_opts. The move/NULL approach I suggested in V1 
happens to work OK, but I think it's non-obvious and could break if we 
ever wanted to use get_replay_opts() more than once - creating separate 
copies reduces the number of surprises.

quoted

Good catches.  sequencer_remove_state() in sequencer.c also has a
free(opts->strategy) call.

To make things even more muddy, we have code like
     replay.strategy = replay.default_strategy;
or
     opts->strategy = opts->default_strategy;
which both will probably work really poorly with the calls to
     free(opts->default_strategy);
     free(opts->strategy);
from sequencer_remove_state().  I suspect we've got a few bugs here...

It's not immediately obvious but I think those are actually safe. 
opts->default_strategy is allocated by sequencer_init_config() so it is 
correct to free it and when we assign it in rebase.c we do

     else if (!replay.strategy && replay.default_strategy) {
         replay.strategy = replay.default_strategy;
         replay.default_strategy = NULL;
     }

so there is no double free.

As mentioned above, ASAN isn't catching any double-frees here (but I 
guess that depends on whether or not you trust the test suite to be 
reasonably testing all permutations).

But it's still good to take note of sequencer_remove_state() free'ing 
opts->strategy, because I almost did manage to add a double free when I 
added a free(options.strategy) to cmd_rebase without also xstrdup'ing 
strategy in get_replay_opts().

There is similar code in builtin/revert.c 
which I think is where your other example came from. I think there is a 
leak in builtin/revert.c though

     if (!opts->strategy && opts->default_strategy) {
         opts->strategy = opts->default_strategy;
         opts->default_strategy = NULL;
     }

     /* do some other stuff */

     /* These option values will be free()d */
     opts->gpg_sign = xstrdup_or_null(opts->gpg_sign);
     opts->strategy = xstrdup_or_null(opts->strategy);

So we copy the default strategy, leaking the original copy from 
sequencer_init_options() if --strategy isn't given on the command line. 
I think it would be simple to fix this by making the copy earlier.

     if (!opts->strategy && opts->default_strategy) {
         opts->strategy = opts->default_strategy;
         opts->default_strategy = NULL;
     } else if (opts->strategy) {
     /* This option will be free()d in sequencer_remove_state() */
         opts->strategy = xstrdup(opts->strategy);
     }

Nice find. I'm noticing a lot of interesting leaks in git's options 
handling, and those leaks also tend to be the trickiest ones to fix (as 
my blunder in the original version of this patch demonstrates :) ).

ATB,

   Andrzej