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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
