[PATCH 3/5] exec: Remove recursion from search_binary_handler
Eric W. Biederman
ebiederm at xmission.com
Mon May 11 14:33:21 UTC 2020
Linus Torvalds <torvalds at linux-foundation.org> writes:
> On Sat, May 9, 2020 at 9:30 PM Tetsuo Handa
> <penguin-kernel at i-love.sakura.ne.jp> wrote:
>> Wouldn't this change cause
>> if (fd_binary > 0)
>> bprm->interp_flags = 0;
>> bprm->interp_data = 0;
>> not to be called when "Search for the interpreter" failed?
> Good catch. We seem to have some subtle magic wrt the fd_binary file
> descriptor, which depends on the recursive behavior.
Yes. I Tetsuo I really appreciate you noticing this. This is exactly
the kind of behavior I am trying to flush out and keep from being
> I'm not seeing how to fix it cleanly with the "turn it into a loop".
> Basically, that binfmt_misc use-case isn't really a tail-call.
I have reservations about installing a new file descriptor before
we process the close on exec logic and the related security modules
closing file descriptors that your new credentials no longer give
you access to logic.
I haven't yet figured out how opening a file descriptor during exec
should fit into all of that.
What I do see is that interp_data is just a parameter that is smuggled
into the call of search binary handler. And the next binary handler
needs to be binfmt_elf for it to make much sense, as only binfmt_elf
(and binfmt_elf_fdpic) deals with BINPRM_FLAGS_EXECFD.
So I think what needs to happen is to rename bprm->interp_data to
bprm->execfd, remove BINPRM_FLAGS_EXECFD and make closing that file
descriptor free_bprm's responsiblity.
I hope such a change will make it easier to see all of the pieces that
are intereacting during exec.
I am still asking: is the installation of that file descriptor useful if
it is not exported passed to userspace as an AT_EXECFD note?
I will dig in and see what I can come up with.
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