[PATCH] x86: Add a GNU_PROPERTY_X86_ISA_1_USED note if needed

[H.J. Lu]

On Fri, Jul 20, 2018 at 8:06 AM, Rich Felker <dalias@libc.org> wrote:
> On Fri, Jul 20, 2018 at 06:38:52AM -0700, H.J. Lu wrote:
>> On Fri, Jul 20, 2018 at 6:31 AM, Rich Felker <dalias@libc.org> wrote:
>> > On Fri, Jul 20, 2018 at 05:24:08AM -0700, H.J. Lu wrote:
>> >> On Fri, Jul 20, 2018 at 2:30 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> >> > On 19/07/18 22:33, H.J. Lu wrote:
>> >> >>
>> >> >> Here is the updated patch.  Any comments?
>> >> >>
>> >> >>
>> >> >> H.J.
>> >> >> --
>> >> >> When -z separate-code, which is enabled by default for Linux/x86, is
>> >> >> used to create executable, ld won't place any data in the code-only
>> >> >> PT_LOAD segment.  If there are no data sections placed before the
>> >> >> code-only PT_LOAD segment, the program headers won't be mapped into
>> >> >> any PT_LOAD segment.  When the executable tries to access it (based
>> >> >> on the program header address passed in AT_PHDR), it will lead to
>> >> >> segfault.  This patch inserts a GNU_PROPERTY_X86_ISA_1_USED note if
>> >> >> there may be no data sections before the text section so that the
>> >> >> first PT_LOAD segment won't be code-only and will contain the program
>> >> >> header.
>> >> >
>> >> >
>> >> > i agree with Rich's comment on the bugzilla ticket:
>> >> > simply always adding a .phdrs section that covers
>> >> > the program headers would ensure that this kind of
>> >> > bug is not introduced on other targets and seems
>> >> > less hackish than adding a note.
>> >>
>> >> We can explore it.
>> >>
>> >> > i don't know why there is no such section already,
>> >> > or if there is any drawback adding such a section,
>> >> > is the requirement to access program headers in a
>> >> > static linked executable a linux only thing?
>> >>
>> >> The program headers aren't required to be mapped into
>> >> process image.  It may not be correct for Linux kermel
>> >> to set AT_PHDR to an unmapped address.  But I'd like
>> >> to support the existing kernel.  I am going to check in
>> >> my change ASIS and backport it to 2.31 branch.
>> >
>> > They are required to be mapped, for at least these reasons:
>> >
>> > 1. PT_TLS is the only way the runtime can find the program's TLS
>> >    image.
>> >
>> > 2. They're exposed as a public interface via dl_iterate_phdr, and the
>> >    AT_PHDR auxv entry.
>>
>> Only this one is relevant here.   The others don't need program headers
>> mapped into process image.
>
> Sure they do. Perhaps you're forgetting static linking. In the
> dynamic-linked case, except for the main program, the dynamic linker
> has access to the file and could load them and store them somewhere
> other than mapped as part of a load segment. But for a static linked
> program there is no access to the file available.
>

Can you show that AT_PHDR isn't used in this case?

-- 
H.J.