GDB python script to print the memory layout of structures.

how-to-use.md

Features

1. Make sure you build the binary with debug symbols. For example: RUSTFLAGS=-g cargo build --release
2. Use rust-gdb to start the binary and load the script: rust-gdb rust_binary -x memlayout-rust.py
3. Break anywhere you like, and use the command mem-layout VARIABLE_NAME to print the layout of that variable:

4. Custom structures are also supported:

struct S1 {
    v1: Vec<i32>,
    b2: u8,
    c3: Box<Vec<usize>>,
}

Known issues

• Some pointers may not be translated correctly. If you see a member's type is 'None' and has size of 0x8, that's a pointer.

memlayout-rust.py

import gdb
# https://stackoverflow.com/questions/9788679/how-to-get-the-relative-address-of-a-field-in-a-structure-dump-c

class Memlayout(gdb.Command):
    def __init__(self):
        super (Memlayout, self).__init__ ('mem-layout', gdb.COMMAND_DATA)
 
    def invoke(self, arg, from_tty):
        argv = gdb.string_to_argv(arg)
        if len(argv) != 1:
            raise gdb.GdbError('mem-layout takes exactly 1 argument.')

        stype = gdb.parse_and_eval(argv[0]).type
        dfs_offset(0, 0, stype, argv[0])
        # print (argv[0], stype, '{')
        # for field in stype.fields():
        #     tpsz = field.bitsize
        #     if tpsz == 0:
        #         tpsz = field.type.sizeof
        #     print ('    [0x%x] %s, size=%x, is_arti=%s, is_basic=%s' % (field.bitpos//8, field.name, tpsz, str(field.artificial), str(field.is_base_class)) )
        # print ('}')

# helper function
def dfs_offset(cur_level, cur_off, cur_type, filed_name = ""):
    try:
        fields = cur_type.fields()
    except TypeError:
        fields = []
    addr = "[0x%03x]"%(cur_off)
    addr = addr.ljust(12, ' ')
    print("%s%s%s %s, size=0x%x " % ( 
        addr, ' '*4*cur_level, cur_type.name, filed_name, cur_type.sizeof
        ), end='')
    if len(fields) != 0:
        print('{')
    else:
        print()
    inner_off = 0
    fields = sorted(fields, key=lambda x: x.bitpos)
    for field in fields:
        dfs_offset(cur_level+1, cur_off+inner_off, field.type, filed_name=field.name)
        tpsz = field.bitsize
        if tpsz == 0:
            tpsz = field.type.sizeof
        inner_off += tpsz
    if len(fields) != 0:
        print("%s%s}" % (' '*12, ' '*4*cur_level))


Memlayout()