Qemu-user : get memory maps while debugging remotely

Question

I am trying to get the memory map of a process I am debugging remotely (peda pull request link), the process is ran with qemu-user, for example:

qemu-arm -L /usr/arm-linux-gnueabihf/ -g 1234 ./ch47

the debugging is done with gdb, commands:

$ gdb-multiarch --nx -q ch47

(gdb) target remote localhost:1234

Remote debugging using localhost:1234
warning: remote target does not support file transfer, attempting to access files from local filesystem.
warning: Unable to find dynamic linker breakpoint function.
GDB will be unable to debug shared library initializers
and track explicitly loaded dynamic code.

(gdb) info inferiors 
  Num  Description       Executable        
* 1    Remote target     /home/redouane/infosec/arm_uaf/ch47 

(gdb) remote get /proc/self/maps /tmp/map
Remote I/O error: Fonction non implantée

The debugged process doesn't have a PID as I see (it's ran in the address-space of qemu-arm, not a separate process).

I am wondering, how does an extension like pwndbg retrieve the memory maps when debugging remotely, and the target does not support file transfer?

pwndbg> vmmap
LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA
   0x10000    0x13000 r-xp     3000 0      /home/redouane/infosec/arm_uaf/ch47
   0x13000    0x22000 ---p     f000 2000   /home/redouane/infosec/arm_uaf/ch47
   0x22000    0x23000 r--p     1000 2000   /home/redouane/infosec/arm_uaf/ch47
   0x23000    0x24000 rw-p     1000 3000   /home/redouane/infosec/arm_uaf/ch47
0xff7c5000 0xff7dd000 r-xp    18000 0      [linker]
0xff7dd000 0xff7ed000 ---p    10000 18000  [linker]
0xff7ed000 0xff7ee000 r--p     1000 18000  [linker]
0xff7ee000 0xff7ef000 rw-p     1000 19000  [linker]
0xfffee000 0xffff0000 rw-p     2000 0      [stack]

The primary way pwndbg does this is via auxv(github.com/pwndbg/pwndbg/blob/…) We also do exploration of registers at every stop to find other pages (github.com/pwndbg/pwndbg/blob/…) — Stoud
– Stoud, Commented Nov 13, 2019 at 6:25

StashOfCode · Accepted Answer · 2022-05-05 10:35:37Z

If you were looking not for the whole memory map, but just the base adress for libc as I used to, you may use the -strace option of QEMU and then read the results.

Here is an example with qemu-mips :

$ qemu-mips -noaslr -nx -strace ./test
...
25631 openat(AT_FDCWD,"/lib/libc.so.6",O_RDONLY|O_CLOEXEC) = 3
25631 read(3,0x7ffff4ac,512) = 512
25631 prctl(46,13,2147480748,512,1928908800,0) = -1 errno=22 (Invalid argument)
25631 _llseek(3,0,520,0x7ffff250,SEEK_SET) = 0
25631 read(3,0x7ffff280,36) = 36
25631 _llseek(3,0,828,0x7ffff228,SEEK_SET) = 0
25631 read(3,0x7ffff258,32) = 32
25631 fstat64(3,0x7ffff360) = 0
25631 mmap2(NULL,1638448,PROT_EXEC|PROT_READ,MAP_PRIVATE|MAP_DENYWRITE,3,0) = 0x7f655000
...

Here, you see that the base address for libc once it has been succesfully located and loaded is 0x7f655000.

For a more complete memory map, you may use the -mmap option, but it won't show what a memory map is for. For example, for the same map :

mmap: start=0x00000000 len=0x00190030 prot=r-x flags=MAP_PRIVATE fd=3 offset=00000000
ret=0x7f655000
start    end      size     prot
00400000-00401000 00001000 r-x
00410000-00412000 00002000 rw-
6ae9c000-6aec0000 00024000 r-x
6aec0000-6aecf000 0000f000 ---
6aecf000-6aed1000 00002000 rw-
7f655000-7f7e6000 00191000 r-x
7f7e6000-7f7fd000 00017000 r--
7f7fd000-7f7ff000 00002000 rw-
7f7ff000-7f800000 00001000 ---
7f800000-00000000 80800000 rw-

Here you see that the file descriptor provided to mmap2 () is 3, but you don't know what file it is.

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