On 1/5/20 5:07 am, Eric W. Biederman wrote:
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
On Thu, Apr 30, 2020 at 7:10 AM Greg Ungerer <gerg@xxxxxxxxxxxxxx> wrote:
Most of that file goes back to pre-git days. And most of the commits
since are not so much about binfmt_flat, as they are about cleanups or
changes elsewhere where binfmt_flat was just a victim.
I'll have a look at this.
Thanks.
Quick hack test shows moving setup_new_exec(bprm) to be just before
install_exec_creds(bprm) works fine for the static binaries case.
Doing the flush_old_exec(bprm) there too crashed out - I'll need to
dig into that to see why.
Just moving setup_new_exec() would at least allow us to then join the
two together, and just say "setup_new_exec() does the credential
installation too".
But it is only half a help if we allow failure points between
flush_old_exec and install_exec_creds.
Greg do things work acceptably if install_exec_creds is moved to right
after setup_new_exec? (patch below)
Yes, confirmed. Worked fine with that patch applied.
Looking at the code in load_flat_file after setup_new_exec it looks like
the kinds of things that in binfmt_elf.c we do after install_exec_creds
(aka vm_map). So I think we want install_exec_creds sooner, instead
of setup_new_exec later.
But if it's true that nobody really uses the odd flat library support
any more and there are no testers, maybe we should consider ripping it
out...
I looked a little deeper and there is another reason to think about
ripping out the flat library loader. The code is recursive, and
supports a maximum of 4 shared libraries in the entire system.
load_flat_binary
load_flat_file
calc_reloc
load_flat_shared_libary
load_flat_file
....
I am mystified with what kind of system can survive with a grand total
of 4 shared libaries. I think my a.out slackware system that I ran on
my i486 had more shared libraries.
The kind of embedded systems that were built with this stuff 20 years
ago didn't have lots of applications and libraries. I think we found
back then that most of your savings were from making libc shared.
Less significant gains from making other libraries shared. And there
was a bit of extra pain in setting them up with the shared library
code generation options (that had to be unique for each one).
The whole mechanism is a bit of hack, and there was a few other
limitations with the way it worked (I don't recall what they were
right now).
I am definitely in favor of removing it.
Regards
Greg
Having read just a bit more it is definitely guaranteed (by the code)
that the first time load_flat_file is called id 0 will be used (aka id 0
is guaranteed to be the binary), and the ids 1, 2, 3 and 4 will only be
used if a relocation includes that id to reference an external shared
library. That part of the code is drop dead simple.
---
This is what I was thinking about applying.
diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
index 831a2b25ba79..1a1d1fcb893f 100644
--- a/fs/binfmt_flat.c
+++ b/fs/binfmt_flat.c
@@ -541,6 +541,7 @@ static int load_flat_file(struct linux_binprm *bprm,
/* OK, This is the point of no return */
set_personality(PER_LINUX_32BIT);
setup_new_exec(bprm);
+ install_exec_creds(bprm);
}
/*
@@ -963,8 +964,6 @@ static int load_flat_binary(struct linux_binprm *bprm)
}
}
- install_exec_creds(bprm);
-
set_binfmt(&flat_format);
#ifdef CONFIG_MMU
