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Date: Thu, 04 Mar 2021 19:18:11 -0800
From: Michael Forney <mforney@...rney.org>
To: musl@...ts.openwall.com
Subject: ld-musl-* and empty .eh_frame

Hi,

√Črico noticed that cproc (my C compiler) produced executables that
musl's dynamic linker fails to load when passed as an argument:

  /lib/ld-musl-x86_64.so.1: ./t: Not a valid dynamic program

However, running ./t directly works fine. It turns out that this
is because the executables have an empty .eh_frame section, which
causes musl to attempt an mmap with length 0 which fails with EINVAL.

GNU ld seems to always create a .eh_frame section in the final
executable (unless you pass --no-ld-generated-unwind-info), regardless
of whether any of the objects had one. Since none of the objects I
built have an .eh_frame and none of musl's crt*.o have one, it ends
up empty.

gcc does not have this problem because its crtend.o has a non-empty
.eh_frame (size is 4, so looks to be a CIE terminator according to LSB[0]).

Here's a short shell session demonstrating the problem:

  $ cat t.s
  .text
  .globl main
  main:
          movl $123, %eax
          ret
  $ as -o t.o t.s
  $ ld --dynamic-linker /lib/ld-musl-x86_64.so.1 -o t /lib/crt1.o /lib/crti.o t.o /lib/libc.so /lib/crtn.o
  $ ./t ; echo $?
  123
  $ /lib/ld-musl-x86_64.so.1 ./t
  /lib/ld-musl-x86_64.so.1: ./t: Not a valid dynamic program
  $ strace /lib/ld-musl-x86_64.so.1 ./t
  execve("/lib/ld-musl-x86_64.so.1", ["/lib/ld-musl-x86_64.so.1", "./t"], 0x7ffd8c17e4e8 /* 34 vars */) = 0
  arch_prctl(ARCH_SET_FS, 0x7f3691752aa8) = 0
  set_tid_address(0x7f3691754fd8)         = 31726
  open("./t", O_RDONLY|O_LARGEFILE)       = 3
  read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\2\0>\0\1\0\0\0 \20@\0\0\0\0\0"..., 960) = 960
  mmap(0x400000, 16384, PROT_READ, MAP_PRIVATE, 3, 0) = 0x400000
  mmap(0x401000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x401000
  mmap(0x402000, 0, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = -1 EINVAL (Invalid argument)
  munmap(0x400000, 16384)                 = 0
  writev(2, ["/lib/ld-musl-x86_64.so.1: ./t: N"...59, NULL0], 2/lib/ld-musl-x86_64.so.1: ./t: Not a valid dynamic program
  ) = 59
  exit_group(1)                           = ?
  +++ exited with 1 +++
  $

This leaves me with a few questions:

1. Is it invalid for an ELF executable to have an empty .eh_frame
   section? The only documentation I could find about it is [0],
   which says that it must contain one or more CFI records, so 0
   would be invalid.
2. Is it the compiler's responsibility to link with an object
   containing a CIE terminator (like gcc's crtend.o) to prevent an
   empty .eh_frame section?
3. Is it a bug that GNU ld creates an empty .eh_frame by default,
   even when none of the objects it is linking have one? It looks
   like lld does not create an .eh_frame in this case.
4. Should musl's ld.so be able to handle such executables? The
   kernel does not seem to have a problem with it, as well glibc's
   ld.so with an executable I crafted with a 0-length .eh_frame
   section.

Or perhaps some combination of the four? Any insight is appreciated.

Thanks!

[0] https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA/ehframechpt.html

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