Potential Memory Leak in libc DNS Resolution Functions

[Florian Weimer]

* Charalampos Mitrodimas via Libc-help:

> I've encountered a potential memory leak in libc, specifically within
> the DNS resolution functions, as identified by Valgrind. The leak
> involves __libc_alloc_buffer_allocate and related functions. Here's the
> Valgrind output snippet:
>
>    ==4151== 156 bytes in 1 blocks are definitely lost in loss record
> 730 of 1,029
>    ==4151==    at 0x4848899: malloc (in
> /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
>    ==4151==    by 0x4D15048: __libc_alloc_buffer_allocate
> (alloc_buffer_allocate.c:26)
>    ==4151==    by 0x4DBDC65: alloc_buffer_allocate (alloc_buffer.h:143)
>    ==4151==    by 0x4DBDC65: __resolv_conf_allocate (resolv_conf.c:391)
>    ==4151==    by 0x4DB8D0A: __resolv_conf_load (res_init.c:599)

This is unfortunately not very illuminating because the struct
resolv_conf functions are reference-counted.  If there's a counter
management bug somewhere, it would be reported like this by valgrind.

> This issue arose in an application using Rust's email library Lettre,
> ultimately traced back to libc during DNS resolution. Could you please
> advise if this is a known issue and recommend any steps to address it?
> I'm willing to contribute a fix, but want to make sure this is of
> concern for the libc team, before starting the investigation.

I don't recall a problem like this being reported before.  Can you
reproduce it on a recent glibc version?  We have some cases of
known/intended leaks (if the memory that can be leaked is bounded for
the life-time of the process), but that doesn't apply here.  It
certainly looks like something we'd want to fix.

Thanks,
Florian