Re: [PATCH v3] ELF: document some de-facto PT_* ABI quirks

[Alexey Dobriyan <adobriyan@gmail.com>]

On Thu, Dec 07, 2023 at 09:03:45AM -0600, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > *thread necromancy* Question below...
> >
> > On Sat, Apr 15, 2023 at 08:37:29PM +0300, Alexey Dobriyan wrote:
> >> Turns out rules about PT_INTERP, PT_GNU_STACK and PT_GNU_PROPERTY
> >> program headers are slightly different.
> >> 
> >> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> >> ---
> >> 
> >> 	v3: move to Documentation/userspace-api/
> >> 	v2: integrate into documentation build system
> >> 
> >>  Documentation/userspace-api/ELF.rst   |   34 ++++++++++++++++++++++++++++++++++
> >>  Documentation/userspace-api/index.rst |    1 +
> >>  2 files changed, 35 insertions(+)
> >> 
> >> new file mode 100644
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/ELF.rst
> >> @@ -0,0 +1,34 @@
> >> +.. SPDX-License-Identifier: GPL-2.0
> >> +
> >> +=================================
> >> +Linux-specific ELF idiosyncrasies
> >> +=================================
> >> +
> >> +Definitions
> >> +===========
> >> +
> >> +"First" program header is the one with the smallest offset in the file:
> >> +e_phoff.
> 
> Confusing e_phoff is the defined location of the array of program
> headers.
> 
> Perhaps the "First" in that array with the lowest e_phnum?
> 
> >> +"Last" program header is the one with the biggest offset in the file:
> >> +e_phoff + (e_phnum - 1) * sizeof(Elf_Phdr).
> 
> Ditto the "Last" in the array with the largest array index.
> 
> I nit pick this because it sounded at first like you were talking about
> p_offset.  Which is a value contained in the program header entry.
> 
> >> +PT_INTERP
> >> +=========
> >> +
> >> +First PT_INTERP program header is used to locate the filename of ELF
> >> +interpreter. Other PT_INTERP headers are ignored (since Linux 2.4.11).
> >> +
> >> +PT_GNU_STACK
> >> +============
> >> +
> >> +Last PT_GNU_STACK program header defines userspace stack executability
> >> +(since Linux 2.6.6). Other PT_GNU_STACK headers are ignored.
> >> +
> >> +PT_GNU_PROPERTY
> >> +===============
> >> +
> >> +ELF interpreter's last PT_GNU_PROPERTY program header is used (since
> >> +Linux 5.8). If interpreter doesn't have one, then the last PT_GNU_PROPERTY
> >> +program header of an executable is used. Other PT_GNU_PROPERTY headers
> >> +are ignored.
> 
> A more interesting property to document is that PT_GNU_PROPERTY must
> precede PT_INTERP in the linux implementation, otherwise we ignore it.
> 
> > Should we perhaps solve some of these in some way? What would folks
> > prefer the behaviors be? (I like to have things been "as expected", but
> > it's not very obvious here for redundant headers...)
> 
> All of these are really headers that should appear only once.

Yes.

> Quite frankly if we are going to do something with this my sense is that
> we should fail the execve with a clear error code as userspace should
> not be doing this, and accepting a malformed executable will hide
> errors, and perhaps hide someone causing problems.

Maybe do it for PT_GNU_PROPERTY which is relatively new.

> I really don't think having multiple copies of these headers with
> different values is something we should encourage.
> 
> It looks like -ELIBBAD is the documented way to fail and report
> a bad file format.

It is obvious you don't know how much will break.

> For PT_GNU_PROPTERTY perhaps we should accept it anywhere, instead of
> silently ignoring it depending upon it's location?
> 
> I thinking change the code to talk one pass through the program headers
> to identify the interesting headers, and then with the interesting
> headers all identified we go do something with them.
> 
> Anyway just my opinion, but that is what it feels like to me.

_Not_ checking for duplicates will result in the simplest and fastest exec.
which is what current code does.