Re: [RFC 0/2] __vdso_findsym

On Sun, Jun 15, 2014 at 11:20 AM, H. Peter Anvin [off-list ref] wrote:

On June 15, 2014 10:40:03 AM PDT, Andy Lutomirski [off-list ref] wrote:

quoted

On Sun, Jun 15, 2014 at 10:05 AM, H. Peter Anvin [off-list ref] wrote:

quoted

On 06/15/2014 07:35 AM, Rich Felker wrote:

quoted

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

quoted

quoted

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

quoted

bytes off static applications is a very weak argument, simply because

it

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is such a small fraction of the enormous cost of a static

application,

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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