Re: [PATCH] read-cache: fix index corruption with index v4

[Thomas Gummerer <t.gummerer@gmail.com>]

On Tue, Sep 5, 2017 at 8:37 PM, Kevin Willford <kewillf@microsoft.com> wrote:
>> From: Thomas Gummerer [mailto:t.gummerer@gmail.com]
>> Sent: Monday, September 4, 2017 4:58 PM

[..]

>> I unfortunately didn't have more time to dig so
>>
>> > As ce->name is however not nul terminated
>>
>> just comes from my memory and from the patch below actually fixing the
>> corruption, so it's really the most likely cause.  Would be great if
>> someone who can remember more about the index could confirm that this
>> is indeed the case.
>>
>
> Digging into this and ce->name IS nul terminated.  The issue comes in when
> the CE_STRIP_NAME is set, which is only set when using a split index.
> This sets the ce->ce_namelen = 0 without changing the actual ce->name buffer.
> When writing the entry for the split index version 4 it was using the first character
> in the ce->name buffer because of the + 1, which obviously isn't correct.    Before
> it was using a newly allocated name buffer from the ondisk struct which was
> allocated based on the ce_namelen of zero.

Thank you very much for digging into this.  That also explains why
only t1700 was
affected, but none of the other tests.  Will update the commit message.

>>  read-cache.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/read-cache.c b/read-cache.c
>> index 40da87ea71..80830ddcfc 100644
>> --- a/read-cache.c
>> +++ b/read-cache.c
>> @@ -2103,7 +2103,9 @@ static int ce_write_entry(git_SHA_CTX *c, int fd, struct
>> cache_entry *ce,
>>               if (!result)
>>                       result = ce_write(c, fd, to_remove_vi, prefix_size);
>>               if (!result)
>> -                     result = ce_write(c, fd, ce->name + common,
>> ce_namelen(ce) - common + 1);
>> +                     result = ce_write(c, fd, ce->name + common,
>> ce_namelen(ce) - common);
>> +             if (!result)
>> +                     result = ce_write(c, fd, "\0", 1);
>
> You could use the padding variable here as well which is used in the < version 4
> ce_write.

Thanks, will do that.

>>
>>               strbuf_splice(previous_name, common, to_remove,
>>                             ce->name + common, ce_namelen(ce) - common);
>> --
>> 2.14.1.480.gb18f417b89
>
> While looking at the code I was wondering if we could get around the
> whole setting ce->ce_namelen to zero bit but that would be much bigger
> patch and possibly introduce other bugs so this seems the appropriate
> fix for now.
>
> Thanks for finding this!

Thanks for the review! Will send an updated patch in a bit.

> Kevin