Re: [PATCH v3 1/3] sparse-checkout: fix segfault on malformed patterns

[Derrick Stolee <stolee@gmail.com>]

On 12/15/2021 3:56 PM, Junio C Hamano wrote:
> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> From: Derrick Stolee <dstolee@microsoft.com>
>>
>> Then core.sparseCheckoutCone is enabled, the sparse-checkout patterns are
>> used to populate two hashsets that accelerate pattern matching. If the user
>> modifies the sparse-checkout file outside of the 'sparse-checkout' builtin,
>> then strange patterns can happen, triggering some error checks.
>>
>> One of these error checks is possible to hit when some special characters
>> exist in a line. A warning message is correctly written to stderr, but then
>> there is additional logic that attempts to remove the line from the hashset
>> and free the data.
> 
> Makes sense.
> 
>> This leads to a segfault in the 'git sparse-checkout
>> list' command because it iterates over the contents of the hashset, which is
>> now invalid.
> 
> Understandable.
> 
>> The fix here is to stop trying to remove from the hashset. Better to leave
>> bad data in the sparse-checkout matching logic (with a warning) than to
>> segfault.
> 
> True, as long as it won't make the situation worse by depending on
> that bad data to further damage working tree data or in-repo data
> when damaged working tree data gets committed.  And "list segfaults
> with freed/NULLed data---so leave the bad ones in to print these bad
> ones" feels OK-ish.  
> 
> As long as the user is not transporting the listed output to another
> repository, which may fall into "making the situation worse"
> category by spreading an existing breakage, that is.
> 
> In other words, this may paper over the segfault, and it may be safe
> only for "sparse-checkout list", but is it safe for other operations
> that actually use this bad data to further affect other things in
> the repository?  If not, I wonder if we want to hard die to lock the
> repository down before the issue is fixed to avoid spreading the
> damage?

You're right. I should do what we do elsewhere in this method and
disable cone mode pattern matching, reverting to the old matching
algorithms. This will clear the hashsets and avoid using them anywhere
else.

>> +test_expect_success 'malformed cone-mode patterns' '
>> +	git -C repo sparse-checkout init --cone &&
>> +	mkdir -p repo/foo/bar &&
>> +	touch repo/foo/bar/x repo/foo/y &&
>> +	cat >repo/.git/info/sparse-checkout <<-\EOF &&
>> +	/*
>> +	!/*/
>> +	/foo/
>> +	!/foo/*/
>> +	/foo/\*/
>> +	EOF
>> +	cat repo/.git/info/sparse-checkout &&
> 
> Stray debugging output?

Thanks.

>> +	git -C repo sparse-checkout list
> 
> And we are happy as long as the command does not segfault, and we do
> not care what the output is.

And I'll be able to test that the 'list' subcommand changes behavior
when cone mode is disabled, in addition to checking stderr for the
correct warnings.

Thanks,
-Stolee