Re: Debian's 32-bit glibc vs AT_SYSINFO

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Sep 25, 2015 at 7:33 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> On Sep 25, 2015 12:03 AM, "Aurelien Jarno" <aurelien@aurel32.net> wrote:
>>
>> On 2015-09-24 15:15, Andy Lutomirski wrote:
>> > On Thu, Sep 24, 2015 at 3:12 PM, Aurelien Jarno <aurelien@aurel32.net>
>> > wrote:
>> > > On 2015-09-24 14:19, Andy Lutomirski wrote:
>> > >> Hi-
>> > >>
>> > >> It looks like 32-bit x86 Debian glibc doesn't use AT_SYSINFO.  This
>> > >> kills syscall performance.  Upstream glibc as well as Fedora's glibc
>> > >> work fine.
>> > >>
>> > >> This test:
>> > >>
>> > >>
>> > >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_compat&id=9d742073d4c0449b262dbe0a3dab4a7c1d831500
>> > >>
>> > >> inadvertently found this issue.
>> > >>
>> > >> Any ideas?  I'm only barely a Debian user -- I have a Debian-based
>> > >> test VM.
>> > >
>> > > By default, the 32-bit glibc on Debian is built for i586, which does
>> > > not
>> > > have the sysenter/syscall instructions. Therefore it doesn't use
>> > > AT_SYSINFO.
>> >
>> > I find this a bit odd.  glibc isn't supposed to have those
>> > instructions, and i586 kernels will still provide AT_SYSINFO (I
>> > think), and it'll still work.  Is this a Debian-specific thing?
>>
>> Yes, i586 kernels provide AT_SYSINFO, but it can't be used by the glibc,
>> as int 0x80 has to be used for syscall on i586 processors. It's nothing
>> specific to Debian, it's like that in upstream glibc. See:
>>
>>
>> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/i386/i686/dl-sysdep.h
>> and
>>
>> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/i386/dl-sysdep.h
>>
>
> OK, that's weird.  I really hope the kernel never had a bug in which
> AT_SYSINFO pointed at SYSENTER on a system that didn't support SYSENTER.
>

Filed upstream:

https://sourceware.org/bugzilla/show_bug.cgi?id=19006