Fix undefined behavior in dsortf comparison function

[visitorckw]

In the dsortf function, when both (*e1)->name[0] == '/' and (*e2)->name[0] == '/', a situation occurs where a < b but b < a, violating the transitivity requirement for comparison functions used by qsort as specified by the C standard. This leads to undefined behavior, which can cause incorrect sorting and memory corruption in glibc's qsort implementation, creating a potential security issue [1].

Fix the issue by returning 0 when both entries begin with '/', preventing the undefined behavior.

[1] https://www.qualys.com/2024/01/30/qsort.txt

[notroj]

Thanks a lot for the PR!

[notroj]

I think this can never happen in practice for mod_autoindex because there will only ever be one entry in the list which meets that test, so it's not a practical concern but I'll merge the fix anyway for safety.

[notroj]

Review from @rpluem on dev@ here - I clearly did not think about this as long as he did 😃

https://lists.apache.org/thread/135z7yn9hjk7rly4y1r1so5rqjymgfx8

[visitorckw]

Thank you for letting me know about this email!

It seems I can't reply directly to this email because I don't have an @apache.org email, and the mailing list server blocks it.
So, I'll leave my message here again:

Hi Ruediger,

On Thu, Jan 09, 2025 at 08:54:13AM +0100, Ruediger Pluem wrote:

I am fine with the change and sorry for getting into a theoretical discussion here, but I think the function before the patch was
transitive (at least regarding to the '/' topic. I have not checked other cases):

Let's assume x = '/', y = '/', z = '/'

dsortf(x, y) == -1
dsortf(y, z) == -1
dsortf(x, z) == -1

Transitive

Thanks for your review!

I initially didn't think this through carefully. However, I'll include
in the explanation that violating transitivity considers the following
case:

x = '/', y = '/', z = '/'

dsortf(x, y) == -1
dsortf(y, z) == -1
dsortf(z, x) == -1

This leads to the conclusion x < y && y < z && z < x, which might also
count as a violation of transitivity since it should ensure z > x ?

Regards,
Kuan-Wei