Re: [PATCH] x86: Add a GNU_PROPERTY_X86_ISA_1_USED note if needed

[Rich Felker <dalias at libc dot org>]

On Fri, Jul 20, 2018 at 08:36:48AM -0700, H.J. Lu wrote:
> 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?

Maybe I'm misunderstanding you then. AT_PHDR is the vector by which
the program headers are found at runtime, but I consider it being part
of a public interface for applications (#2 in my list) to be a
separate issue from it being necessary to runtime implementation
internals (all the other points).

Rich