Re: [PATCH RFC] binfmt_elf: fully allocate bss pages

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Sun, 24 Sep 2023, Eric W. Biederman wrote:
Sebastian Ott <sebott@xxxxxxxxxx> writes:

Hej,

since we figured that the proposed patch is not going to work I've spent a
couple more hours looking at this (some static binaries on arm64 segfault
during load [0]). The segfault happens because of a failed clear_user()
call in load_elf_binary(). The address we try to write zeros to is mapped with
correct permissions.

After some experiments I've noticed that writing to anonymous mappings work
fine and all the error cases happend on file backed VMAs. Debugging showed that
in elf_map() we call vm_mmap() with a file offset of 15 pages - for a binary
that's less than 1KiB in size.

Looking at the ELF headers again that 15 pages offset originates from the offset
of the 2nd segment - so, I guess the loader did as instructed and that binary is
just too nasty?

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x0000000000000178 0x0000000000000178  R E    0x10000
  LOAD           0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8
                 0x0000000000000000 0x0000000000000008  RW     0x10000
  NOTE           0x0000000000000120 0x0000000000400120 0x0000000000400120
                 0x0000000000000024 0x0000000000000024  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x10

As an additional test I've added a bunch of zeros at the end of that binary
so that the offset is within that file and it did load just fine.

On the other hand there is this section header:
  [ 4] .bss              NOBITS           000000000041ffe8  0000ffe8
       0000000000000008  0000000000000000  WA       0     0     1

"sh_offset
This member's value gives the byte offset from the beginning of the file to
the first byte in the section. One section type, SHT_NOBITS described
below, occupies no space in the file, and its sh_offset member locates
the conceptual placement in the file.
"

So, still not sure what to do here..

Sebastian

[0] https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@xxxxxxxxxx/

I think that .bss section that is being generated is atrocious.

At the same time I looked at what the linux elf loader is trying to do,
and the elf loader's handling of program segments with memsz > filesz
has serious remnants a.out of programs allocating memory with the brk
syscall.

Lots of the structure looks like it started with the assumption that
there would only be a single program header with memsz > filesz the way
and that was the .bss.   The way things were in the a.out days and
handling of other cases has been debugged in later.

So I have modified elf_map to always return successfully when there is
a zero filesz in the program header for an elf segment.

Then I have factored out a function clear_tail that ensures the zero
padding for an entire elf segment is present.

Please test this and see if it causes your test case to work.

Sadly, that causes issues for other programs:

[   44.164596] Run /init as init process
[   44.168763] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
[   44.176409] CPU: 32 PID: 1 Comm: init Not tainted 6.6.0-rc2+ #89
[   44.182404] Hardware name: GIGABYTE R181-T92-00/MT91-FS4-00, BIOS F34 08/13/2020
[   44.189786] Call trace:
[   44.192220]  dump_backtrace+0xa4/0x130
[   44.195961]  show_stack+0x20/0x38
[   44.199264]  dump_stack_lvl+0x48/0x60
[   44.202917]  dump_stack+0x18/0x28
[   44.206219]  panic+0x2e0/0x350
[   44.209264]  do_exit+0x370/0x390
[   44.212481]  do_group_exit+0x3c/0xa0
[   44.216044]  get_signal+0x800/0x808
[   44.219521]  do_signal+0xfc/0x200
[   44.222824]  do_notify_resume+0xc8/0x418
[   44.226734]  el0_da+0x114/0x120
[   44.229866]  el0t_64_sync_handler+0xb8/0x130
[   44.234124]  el0t_64_sync+0x194/0x198
[   44.237776] SMP: stopping secondary CPUs
[   44.241740] Kernel Offset: disabled
[   44.245215] CPU features: 0x03000000,14028142,10004203
[   44.250342] Memory Limit: none
[   44.253383] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---




[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [NTFS 3]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [NTFS 3]     [Samba]     [Device Mapper]     [CEPH Development]

  Powered by Linux