Subject: Re: [PATCH] pull, fetch: fix segfault in --set-upstream option

From: Junio C Hamano <[email-masked]>
Date: Mon, 19 Jul 2021 08:17:26 -0700

Ævar Arnfjörð Bjarmason  <[email-masked]> writes:

> Fix a segfault in the --set-upstream option added in
> 24bc1a12926 (pull, fetch: add --set-upstream option, 2019-08-19) added
> in v2.24.0.
>
> The code added there did not do the same checking we do for "git
> branch" itself since 8efb8899cfe (branch: segfault fixes and
> validation, 2013-02-23), which in turn fixed the same sort of segfault
> I'm fixing now in "git branch --set-upstream-to", see
> 6183d826ba6 (branch: introduce --set-upstream-to, 2012-08-20).
>
> The error message I'm adding here is an amalgamation of the error
> added for "git branch" in 8efb8899cfe, and the error output
> install_branch_config() itself emits, i.e. it trims "refs/heads/" from
> the name and says "branch X on remote", not "branch refs/heads/X on
> remote".
>
> Reported-by: Clemens Fruhwirth <[email-masked]>
> Reported-by: Jan Pokorný <[email-masked]>
> Signed-off-by: Ævar Arnfjörð Bjarmason <[email-masked]>

Hmph, there is 5e0fa8cb (fetch: fix segfault on --set-upstream while
on a detached HEAD, 2021-07-06) from Clemens, which warns and skips
instead of dies.  Given that a handful of other conditions in this
function results in warnings "not setting upstream due to X", it
might be more in line with the spirit of the current system (it is
debatable if we should stress the fact that we did not set the
configuration with hard dying, though).

> ---
>
> On Mon, Jul 19 2021, Jan Pokorný wrote:
>
>> Hello,
>>
>> Clemens Fruhwirth píše v Po 05. 07. 2021 v 17:46 +0200:
>>> What did you do before the bug happened? (Steps to reproduce your
>>> issue)
>>> 
>>> Run "git pull origin nixos-unstable --rebase --set-upstream"
>>> on a repo that had no branch set, e.g. when running "git branch" gave
>>> "* (no branch)"
>>> 
>
> Thanks for the report both & sorry that this fell through the cracks
> for so long.
>
> The bug itself seems rather obvious given the benefit of that
> reproduction scenario & backtrace, as noted above we've been playing
> whack-a-mole with a related segfault since 2012.
>
> It would be nice if you could test this patch and confirm, but given
> the trivality of the segfault it shouldn't be necessary, I'm confident
> that this fixes the bug you two reported. Thanks both!
>
>  builtin/fetch.c         | 11 +++++++++++
>  t/t5553-set-upstream.sh | 22 ++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
>
> diff --git a/builtin/fetch.c b/builtin/fetch.c
> index 9191620e50c..c2eac8f15e5 100644
> --- a/builtin/fetch.c
> +++ b/builtin/fetch.c
> @@ -1623,6 +1623,17 @@ static int do_fetch(struct transport *transport,
>  			}
>  		}
>  		if (source_ref) {
> +			if (!branch) {
> +				const char *shortname = NULL;
> +				if (!skip_prefix(source_ref->name,
> +						 "refs/heads/", &shortname))
> +					shortname = source_ref->name;
> +				    
> +				die(_("could not set upstream of HEAD to '%s' from '%s' when "
> +				      "it does not point to any branch."),
> +				    shortname, transport->remote->name);
> +			}
> +
>  			if (!strcmp(source_ref->name, "HEAD") ||
>  			    starts_with(source_ref->name, "refs/heads/"))
>  				install_branch_config(0,
> diff --git a/t/t5553-set-upstream.sh b/t/t5553-set-upstream.sh
> index b1d614ce18c..ae90554645e 100755
> --- a/t/t5553-set-upstream.sh
> +++ b/t/t5553-set-upstream.sh
> @@ -91,6 +91,17 @@ test_expect_success 'fetch --set-upstream with valid URL sets upstream to URL' '
>  	check_config_missing other2
>  '
>  
> +test_expect_success 'fetch --set-upstream with a detached HEAD' '
> +	git checkout HEAD^0 &&
> +	test_when_finished "git checkout -" &&
> +	cat >expect <<-\EOF &&
> +	fatal: could not set upstream of HEAD to '"'"'main'"'"' from '"'"'upstream'"'"' when it does not point to any branch.
> +	EOF
> +	test_must_fail git fetch --set-upstream upstream main 2>actual.raw &&
> +	grep ^fatal: actual.raw >actual &&
> +	test_cmp expect actual
> +'
> +
>  # tests for pull --set-upstream
>  
>  test_expect_success 'setup bare parent pull' '
> @@ -178,4 +189,15 @@ test_expect_success 'pull --set-upstream with valid URL and branch sets branch'
>  	check_config_missing other2
>  '
>  
> +test_expect_success 'pull --set-upstream with a detached HEAD' '
> +	git checkout HEAD^0 &&
> +	test_when_finished "git checkout -" &&
> +	cat >expect <<-\EOF &&
> +	fatal: could not set upstream of HEAD to '"'"'main'"'"' from '"'"'upstream'"'"' when it does not point to any branch.
> +	EOF
> +	test_must_fail git pull --set-upstream upstream main 2>actual.raw &&
> +	grep ^fatal: actual.raw >actual &&
> +	test_cmp expect actual
> +'
> +
>  test_done