Re: [RFC 0/2] __vdso_findsym

["H. Peter Anvin" <hpa@zytor.com>]

On June 15, 2014 10:40:03 AM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Sun, Jun 15, 2014 at 10:05 AM, H. Peter Anvin <hpa@zytor.com> 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.

-- 
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