Re: [PATCH v2] scalar: avoid segfault in reconfigure --all

[Patrick Steinhardt <ps@pks.im>]

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On Sun, May 05, 2024 at 01:58:18AM +0000, Derrick Stolee via GitGitGadget wrote:
> From: Derrick Stolee <stolee@gmail.com>
> 
> During the latest v2.45.0 update, 'scalar reconfigure --all' started to
> segfault on my machine. Breaking it down via the debugger, it was
> faulting on a NULL reference to the_hash_algo, which is a macro pointing
> to the_repository->hash_algo.
> 
> This NULL reference appears to be due to the way the loop is abusing the
> the_repository pointer, pointing it to a local repository struct after
> discovering that the current directory is a valid Git repository. This
> repo-swapping bit was in the original implementation from 4582676075
> (scalar: teach 'reconfigure' to optionally handle all registered
> enlistments, 2021-12-03), but only recently started segfaulting while
> trying to parse the HEAD reference. This also only happens on the
> _second_ repository in the list, so does not reproduce if there is only
> one registered repo.

I think this explanation could use an update now that we have figured
out the root cause likely being a detached HEAD, where `get_oid_hex()`
will then access a NULL pointer.

> My first inclination was to try to refactor cmd_reconfigure() to execute
> 'git for-each-repo' instead of this loop. In addition to the difficulty
> of executing 'scalar reconfigure' within 'git for-each-repo', it would
> be difficult to perform the clean-up logic for non-existent repos if we
> relied on that child process.
> 
> Instead, I chose to move the temporary repo to be within the loop and
> reinstate the_repository to its old value after we are done performing
> logic on the current array item.
> 
> Add tests to t9210-scalar.sh to test 'scalar reconfigure --all' with
> multiple registered repos. There are two different ways that the old
> use of the_repository could trigger bugs. These issues are being solved
> independently to be more careful about the_repository being
> uninitialized, but the change in this patch around the use of
> the_repository is still a good safety precaution.
> 
> Co-authored-by: Patrick Steinhardt <ps@pks.im>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> Signed-off-by: Derrick Stolee <stolee@gmail.com>
> ---
>     scalar: avoid segfault in reconfigure --all
>     
>     Update 'scalar reconfigure --all' to be more careful with the_repository
>     pointer, avoiding sudden halts in some scenarios.
>     
>     ------------------------------------------------------------------------
>     
>     I noticed this while validating the v2.45.0 release (specifically the
>     microsoft/git fork, but this applies to the core project).
>     
>     Thanks, Patrick, for digging in and finding the critical reasons why
>     this issue can happen. I've included Patrick's test cases and given him
>     co-authorship. I forged his sign-off, so could you please ACK that
>     sign-off, Patrick?
>     
>     -Stolee

Other than the above nit regarding the root cause analysis this patch
looks good to me, and I'm fine with the forged SOB. Thanks!

Patrick

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