[LLVMdev] [llvm-commits] Dealing with a corrupted /proc/self/exe link

[Benjamin Kramer]

On 13.07.2012, at 21:39, Gabor Greif <gabor.greif at alcatel-lucent.com> wrote:

> Benjamin Kramer wrote:
>> On 13.07.2012, at 09:46, Gabor Greif <gabor.greif at alcatel-lucent.com> wrote:
>> 
>>> Hi all,
>>> 
>>> I am in charge of the controlled introduction of clang into
>>> our builds at my workplace. Since all our tools must run from
>>> a ClearCase view for automatic dependency tracking, we have been
>>> biten by a Linux bug, and readlink("/proc/self/exe", ...) gives
>>> nonsensical results. So we need to introduce a configure option
>>> for disallowing this method of executable discovery (the other
>>> one works well).
>> 
>> Interesting, can you describe the linux bug? Are the kernel devs aware of it?
> 
> It is fixed in newer RHEL kernels (>=6). What I know is that this is a
> ClearCase VFS-related bug that fails to do a reverse mapping to obtain
> the logical pathname from the real (into the backing store of ClearCase)
> one.
> 
> Here is a bug report:
> <http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6189256>
> 
>> 
>> We often had reports about /proc/self/exe not working (and thus clang crashing)
>> in chrooted environments. It is possible to mount /proc into the chroot but this
>> seems to be missing from many setups. The code in LLVM that uses /proc/self/exe
>> returns an empty string on error which confuses clang.
> 
> There is no empty string for me, and the returned string is a real object
> (bytewise identical to the real thing) :
> 
> $ cd <into a dynamic view>
> $ cp /bin/ls .
> $ ls -l /proc/self/exe
> lrwxrwxrwx 1 ggreif ocs 0 Jul 13 21:27 /proc/self/exe -> /bin/ls
> $ ./ls -l /proc/self/exe
> lrwxrwxrwx 1 ggreif ocs 0 Jul 13 21:27 /proc/self/exe -> /vol/ocs_userviews25_13/ggreif-hc_stm-OCSnb28718.vws/.s/00056/800006ba4fdf647els
> 
> $ diff ./ls /vol/ocs_userviews25_13/ggreif-hc_stm-OCSnb28718.vws/.s/00056/800006ba4fdf647els
> <no diffs>
> 
> Unfortunately starting from the clang executable, there is no useful
> directory structure to be discovered :-(
> 
>> 
>> I don't really like having an autoconf switch for this as long as you can determine
>> whether the result from /proc/self/exe is valid. When you're adding a fallback to
>> Path.inc anyways, why not just try reading /proc/self/exe first, and if it fails, use
>> your fallback? That would also fix the chroot problem.
> 
> This is not a chroot problem. As shown above, I do not get a valid clang path
> to manipulate and discover include directories, etc.
> 
> The other method in lib/Support/Unix/Path.inc (i.e. dladdr, realpath) works.
> 
> I still maintain that I need the configure option.

Sorry for being mean, but this is a workaround for a bug in the linux kernel that was
fixed years ago and is only visible when using an obscure revision control system.

Also it requires rebuilding LLVM, so the fix isn't even helpful without researching the
issue (if someone else hits it).

With this in mind I really don't see why this has to be in the public tree, requiring
additions to two build systems. Can't you just apply the one-line-patch to Path.inc
locally?

- Ben