Re: [RFC 0/2] __vdso_findsym

[Andy Lutomirski]

On Sun, Jun 15, 2014 at 11:20 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
>
> On June 15, 2014 10:40:03 AM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>On Sun, Jun 15, 2014 at 10:05 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>>> On 06/15/2014 07:35 AM, Rich Felker wrote:
>>>>
>>>> Arguably, it was a mistake for the kernel to expose a virtual ELF to
>>>> begin with, and it should just have exposed a "lookup function by
>>>> name" operation to begin with. Yes this can be done in userspace,
>>but
>>>> I see it more as a matter of "fixing a broken API design".
>>>>
>>>
>>> What the fsck are you smoking?  There is immense value in providing a
>>> stable and very well-defined data structure, which also happens to be
>>> what dynamic linkers already want to consume.  Providing a helper for
>>> crippled libc applications has potential value.  Shaving a few
>>hundred
>>> bytes off static applications is a very weak argument, simply because
>>it
>>> is such a small fraction of the enormous cost of a static
>>application,
>>> and static applications are problematic in a number of other ways,
>>> especially the lack of ability to fix bugs.
>>>
>>> Treating the kernel as an ersatz dynamic library for "static"
>>> applications is kind of silly -- after all, why not provide an entire
>>> libc in the vdso?  I have actually seen people advocate for doing
>>that.
>>
>>To be clear, I have no desire whatsoever to give the vdso an actual
>>ELF parser or anything else that userspace should be providing itself.
>>I think that a special-purpose vdso parser in the vdso makes some
>>sense, though, since userspace might otherwise provide one for the
>>sole purpose of parsing the vdso.
>>
>>And there's plenty of reasons that having the vdso be an ELF image is
>>useful.  For one thing, gdb can take advantage of it.  For another,
>>CRIU is parsing it for a rather different reason, and something like
>>__vdso_findsym won't fill that need.
>>
>>Also, given the general lack of a comprehensible specification of what
>>the GNU flavor of the ELF format actually is [1], there's something to
>>be said for reducing the proliferation of ELF parsers.  glibc and
>>binutils are quite unlikely to become incompatible with each other,
>>but I sincerely doubt that anyone from binutils land is likely to
>>review (and maintain!) my ELF parser, Go's, or a hypothetical future
>>ELF parser from any of the other glibc-less things.  If those things
>>use one that's in the kernel, then it's easy for the kernel to
>>guarantee that each vdso image can successfully parse itself.
>>
>>[1] The only comprehensible description of the GNU hash extension that
>>I could find is on Oracle's blog (!)
>>
>
> Curious about this blog.  We do have a GNU hash implementation in Syslinux, too, for another reference.
>

https://blogs.oracle.com/ali/entry/gnu_hash_elf_sections

FWIW, I bet that __vdso_findsym could be smaller if it used the GNU
hash.  Maybe it would save about the same amount of space that turning
on the GNU hash would take up.

--Andy
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