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Re: Can't execute any MIPS binary

To: Matteo Croce <rootkit85@yahoo.it>
Subject: Re: Can't execute any MIPS binary
From: David Daney <ddaney@avtrex.com>
Date: Tue, 12 Feb 2008 15:54:21 -0800
Cc: linux-mips@linux-mips.org
In-reply-to: <200802130034.25052.rootkit85@yahoo.it>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <200802130034.25052.rootkit85@yahoo.it>
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Matteo Croce wrote:
Hi,
I have a machine, an AR7 MIPS router I want to hack, but I'm unable
to run _any_ executable on that machine outside the ones in the firmware.
I tried building a static mips1 binary, but it fails so:

# /var/test.bin
/var/test.bin: 1: Syntax error: "(" unexpected

so I downloaded a binary builtin in the firmware and I compared it to my own:

$ file busybox.bin test.bin
busybox.bin: ELF 32-bit LSB executable, MIPS, MIPS-I version 1 (SYSV), 
dynamically linked (uses shared libs), stripped
test.bin:    ELF 32-bit LSB executable, MIPS, version 1 (SYSV), statically 
linked, stripped

busybox.bin is the builtin busybox while test.bin is a static HelloWorld

I ran readelf on it:

$ readelf -h busybox.bin
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           MIPS R3000
  Version:                           0x1
  Entry point address:               0x4037e0
  Start of program headers:          52 (bytes into file)
  Start of section headers:          337304 (bytes into file)
  Flags:                             0x5, noreorder, cpic, mips1
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         6
  Size of section headers:           40 (bytes)
  Number of section headers:         21
  Section header string table index: 20
$ readelf -h test.bin
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           MIPS R3000
  Version:                           0x1
  Entry point address:               0x400140
  Start of program headers:          52 (bytes into file)
  Start of section headers:          11780 (bytes into file)
  Flags:                             0x50001007, noreorder, pic, cpic, o32, 
mips32
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         3
  Size of section headers:           40 (bytes)
  Number of section headers:         17
  Section header string table index: 16
$ diff -u <(readelf -h busybox.bin) <(readelf -h test.bin)
--- /dev/fd/63  2008-02-13 00:26:48.880261477 +0100
+++ /dev/fd/62  2008-02-13 00:26:48.880261477 +0100
@@ -8,13 +8,13 @@
   Type:                              EXEC (Executable file)
   Machine:                           MIPS R3000
   Version:                           0x1
-  Entry point address:               0x4037e0
+  Entry point address:               0x400140
   Start of program headers:          52 (bytes into file)
-  Start of section headers:          337304 (bytes into file)
-  Flags:                             0x5, noreorder, cpic, mips1
+  Start of section headers:          11780 (bytes into file)
+  Flags:                             0x50001007, noreorder, pic, cpic, o32, 
mips32
   Size of this header:               52 (bytes)
   Size of program headers:           32 (bytes)
-  Number of program headers:         6
+  Number of program headers:         3
   Size of section headers:           40 (bytes)
-  Number of section headers:         21
-  Section header string table index: 20
+  Number of section headers:         17
+  Section header string table index: 16

The router firmware uses:
# cat /proc/version
Linux version 2.4.17_mvl21-malta-mips_fp_le (root@localhost.localdomain) (gcc 
version 2.95.3 20010315 (release/MontaVista)) #1 Fri Mar 18 11:00:12 EST 2005


IIRC, the definitions of the elf flags were wrong in 2.4.x (x < 20 or 25 or something like that). I had similar problems.

You should be able to run the binary if run a binary editor on it and clear the mips32 flag (i.e. change the flags from 0x50001007 to just 0x1007).

If you want to generate the program with out the mips32 flag in the first place you have to compile everything (libc, libgcc crt*.o, program ) with -mips1

It may be easier to run a program that post processes the executable to clear the flag.


David Daney

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