Re: [PATCH] show-branch: fix SEGFAULT when `--current` and `--reflog` together

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, Apr 19 2022, Gregory David wrote:

> If run `show-branch` with `--current` and `--reflog` simultaneously, a
> SEGFAULT appears.
>
> The bug is that we read over the end of the `reflog_msg` array after
> having `append_one_rev()` for the current branch without supplying a
> convenient message to it.
>
> It seems that it has been introduced in:
> Commit 1aa68d6735 (show-branch: --current includes the current branch.,
> 2006-01-11)
>
> Signed-off-by: Gregory DAVID <gregory.david@p1sec.com>
> Thanks-to: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> ---
>  builtin/show-branch.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/builtin/show-branch.c b/builtin/show-branch.c
> index e12c5e80e3..892241ce0d 100644
> --- a/builtin/show-branch.c
> +++ b/builtin/show-branch.c
> @@ -812,6 +812,26 @@ int cmd_show_branch(int ac, const char **av, const
> char *prefix)
>  		}

Hi. Thanks for sticking with this.

First, your patch is corrupt, I think because your mailer is wrapping it
for you. The Documentation/SubmittingPatches has some advice on how to
use "git format-patch/send-email" directly.

It would be great to have a test case for the existing segfault, is it
sometihng you think you could add to t/t3202-show-branch.sh?

>  		if (!has_head) {
>  			const char *name = head;
> +			struct object_id oid;
> +			char *ref;
> +			unsigned int flags = 0;
> +			char *log_msg;
> +			char *end_log_msg;
> +			timestamp_t timestamp;
> +			int tz;
> +
> +			if (!dwim_ref(*av, strlen(*av), &oid, &ref, 0))
> +				die(_("no such ref %s"), *av);
> +			if(read_ref_at(get_main_ref_store(the_repository),

Please use the usual coding style & one in that file, e.g. "if (" not
"if(". 

> +					ref, flags, 0, i, &oid, &log_msg,
> +					&timestamp, &tz, NULL)) {
> +				end_log_msg = strchr(log_msg, '\n');
> +				if (end_log_msg)
> +					*end_log_msg = '\0';
> +			}

Most of this code is copied from a few lines above where we do much the
same for another case, I wonder if it's worth it to combine the two into
a helper.

> +			if(log_msg == 0 || *log_msg == '\0')

if (!log_msg || !*log_msg) instead, but I see some of this is using a
pattern copied from above...

> +				log_msg = xstrfmt("(none)");
> +			reflog_msg[ref_name_cnt] = xstrfmt("(%s) (current) %s",
> show_date(timestamp, tz, DATE_MODE(RELATIVE)), log_msg);


This code already leaks memory, but we can avoid this case easily by
making that "(none)" part of the second xstrfmt()'s argument list
(i.e. as a ternary).

>  			skip_prefix(name, "refs/heads/", &name);
>  			append_one_rev(name);
>  		}