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[PATCH] x86: Add a GNU_PROPERTY_X86_ISA_1_USED note if needed

Rich Felker dalias@libc.org
Fri Jul 20 13:38:00 GMT 2018 
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.

3. PT_GNU_EH_FRAME is the way (or at least one of the ways) to find
   unwind information.

4. PT_LOAD segments are needed for dladdr to determine if an address
   is within the bounds of an object's memory.

5. PT_GNU_RELRO is necessary to mprotect the relro zone read-only
   after processing relocations.

6. PT_DYNAMIC is the way to find _DYNAMIC in a dynamic-linked program
   and the cleanest way to find the load address for static PIE
   programs.

It's true that *some* of the things in the program headers are mainly
needed prior to passing control from the kernel into the mapped
program, but *a lot* are needed at runtime to implement important
features.

Some of them could be replaced for static linking with auto-generated
symbols, but that's not existing practice, and it's probably good that
it's not since compilers will tend to make transformations that
invalidate start/end symbol hacks.

Rich
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