Re: [musl] $ORIGIN rpath expansion without /proc: code looks wrong

["Érico Nogueira" <ericonr@disroot.org>]

On Wed Nov 17, 2021 at 11:04 AM -03, Alexander Sosedkin wrote:
> Hello, I've encountered a case of a binary with an rpath of
> /some/meaningful/lib:$ORIGIN/../lib
> not starting up due to not finding /some/meaningful/lib/libxyz.so.
> ldd'ing said it's there though.
> And the library was found alright when I patchelf'd rpath to just
> /some/meaningful/lib
>
> I dug into musl code and came across that bit that checks /proc.
> Sure enough, when I tried mounting /proc, it started working fine.
> Yet the error handling from accessing /proc puzzles me:
>
> ldso/dynlink.c, fixup_rpath():
> l = readlink("/proc/self/exe", buf, buf_size);
> if (l == -1) switch (errno) {
> case ENOENT:
> case ENOTDIR:
> case EACCES:
> break;
> default:
> return -1;
> }
> if (l >= buf_size)
> return 0;
> buf[l] = 0;
> origin = buf;
>
> hitting that break like I had means zeroing buf[-1], right?

No. Because `l` is size_t (unsigned long), it's the biggest possible
value for size_t, and `l >= buf_size` will be true, which means the
function returns 0. This conditional also catches the case where
truncation happens in readlink(3).

Documenting this in a comment or changing `break;` for `return 0;` might
make sense, though.

> Could somebody take a look at this and double-check that
> this codepath makes sense?

It does, but it might not be as robust as you wish. fixup_rpath() treats
the RPATH entry as a single string, and does all $ORIGIN substitutions
in one go (what splits the string by ":" is open_path()). This means
that the entire RPATH entry containing $ORIGIN will be ignored if
/proc/self/exe can't be accessed, despite one or more of them not
depending on $ORIGIN.

> My attempts at comprehending it fail irrecoverably at this line.
>
> (CC me on replies, please.
> No nice context to provide, building my own toolchain at
> https://github.com/t184256/bootstrap-from-tcc)