On Thu, Dec 07, 2023 at 09:03:45AM -0600, Eric W. Biederman wrote: > Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxx> > >> --- > >> > >> 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.
