patchmerge-ort: fix calling merge_finalize() with no intermediate merge

From: Elijah Newren <newren@gmail.com>

If some code sets up the data structures for a merge, but then never actually performs one before calling merge_finalize(), then merge_finalize() wouldn't notice that result->priv was NULL and return early, resulting in following that NULL pointer and getting a segfault. There is currently no code in the git codebase that does this, but this issue was found during testing of some proposed patches that had the following structure:

    struct merge_options merge_opt;
    struct merge_result result;

    init_merge_options(&merge_opt, the_repository);
    memset(&result, 0, sizeof(result));

    <do N merges, for some value of N>

    merge_finalize(&merge_opt, &result);

where some flags could cause the code to have N=0, i.e. doing no merges. Add a check for result->priv being NULL and return early to avoid a segfault in these kinds of cases.

While at it, ensure the FREE_AND_NULL() in the function does something useful with the nulling aspect, namely sets result->priv to NULL rather than a mere temporary.

Reported-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
    merge-ort: fix calling merge_finalize() with no intermediate merge
    
    See
    https://lore.kernel.org/git/CABPp-BHCdjOutYqdMO1NbYKNA0BgkXRgwUEKK=MX0kXM-5G_DQ@mail.gmail.com/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1518%2Fnewren%2Ffix-merge-finalize-with-no-merge-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1518/newren/fix-merge-finalize-with-no-merge-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1518

 merge-ort.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

On 4/20/2023 3:14 AM, Elijah Newren via GitGitGadget wrote:

Good call. It also makes the code look better.
 

Perhaps this would be better as

	if (result->priv) {
		clear_or_reinit_internal_opts(result->priv, 0);
		FREE_AND_NULL(result->priv);
	}

to avoid an accidental addition of code to the end of this method that doesn't depend on result->priv?

Thanks, -Stolee

Derrick Stolee <derrickstolee@github.com> writes:

And more correct, if the NULLness of result->priv matters.

It does sound like a good idea.

Thanks.

On Thu, Apr 20, 2023 at 6:10 AM Derrick Stolee <derrickstolee@github.com> wrote:

Ooh, yes, that'd be a good improvement; thanks.

"Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:

Clearly written. The FREE_AND_NULL() bit is a bit embarrassing slip-up for everybody who reviewed and applied the initial patch X-<.

And we need to undo the attribute direction change in merge_start() even if we didn't perform any, so the early return cannot be done before that. So everything makes sense as a fix to the reported problem.

From: Elijah Newren <newren@gmail.com>

If some code sets up the data structures for a merge, but then never actually performs one before calling merge_finalize(), then merge_finalize() wouldn't notice that result->priv was NULL and return early, resulting in following that NULL pointer and getting a segfault. There is currently no code in the git codebase that does this, but this issue was found during testing of some proposed patches that had the following structure:

    struct merge_options merge_opt;
    struct merge_result result;

    init_merge_options(&merge_opt, the_repository);
    memset(&result, 0, sizeof(result));

    <do N merges, for some value of N>

    merge_finalize(&merge_opt, &result);

where some flags could cause the code to have N=0, i.e. doing no merges. Add a check for result->priv being NULL and return early to avoid a segfault in these kinds of cases.

While at it, ensure the FREE_AND_NULL() in the function does something useful with the nulling aspect, namely sets result->priv to NULL rather than a mere temporary.

Reported-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
    merge-ort: fix calling merge_finalize() with no intermediate merge
    
    Changes since v1:
    
     * Moved code into an if-block instead of returning early, as suggested
       by Stolee.
    
    See
    https://lore.kernel.org/git/CABPp-BHCdjOutYqdMO1NbYKNA0BgkXRgwUEKK=MX0kXM-5G_DQ@mail.gmail.com/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1518%2Fnewren%2Ffix-merge-finalize-with-no-merge-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1518/newren/fix-merge-finalize-with-no-merge-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1518

Range-diff vs v1:

 1:  e636027f60c ! 1:  2cb1c63f05b merge-ort: fix calling merge_finalize() with no intermediate merge
     @@ merge-ort.c: void merge_switch_to_result(struct merge_options *opt,
       
      -	clear_or_reinit_internal_opts(opti, 0);
      -	FREE_AND_NULL(opti);
     -+	if (!result->priv)
     -+		return;
     -+	clear_or_reinit_internal_opts(result->priv, 0);
     -+	FREE_AND_NULL(result->priv);
     ++	if (result->priv) {
     ++		clear_or_reinit_internal_opts(result->priv, 0);
     ++		FREE_AND_NULL(result->priv);
     ++	}
       }
       
       /*** Function Grouping: helper functions for merge_incore_*() ***/

 merge-ort.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

On 4/22/2023 4:22 PM, Elijah Newren via GitGitGadget wrote:

v2 LGTM. Thanks!

-Stolee