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Date: Wed, 17 Nov 2021 20:25:11 +0100
From: Alexander Sosedkin <monk@...oiled.info>
To: √Črico Nogueira <ericonr@...root.org>,
 musl@...ts.openwall.com
Subject: Re: $ORIGIN rpath expansion without /proc: code looks wrong

On 11/17/21 18:00, √Črico Nogueira wrote:
> 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,

Oh! Thanks a lot, that's what confused me. Sorry for the noise then.

> 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.

Yeah, I'd say a `return 0;` there would've been easier to comprehend.
I don't think there's much need for a comment...

>> 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.

... because if running /proc-less isn't supported in general,
then such separate expansions are probably not worth the effort.
And it's far from being the only mention of /proc in the code,
so I'm going to presume it's not.

>> 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)

Thanks for the answer, appreciated.

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