Difference between revisions of "Gdb"

Revision as of 23:18, 17 September 2021

2021-09-17 Debugging Zephyr applications:

• https://docs.zephyrproject.org/latest/guides/west/build-flash-debug.html#west-build-flash-debug

How to have gdb stop on access to a memory location within a range of memory addresses, popular solution mentions use of valgrind:

• https://stackoverflow.com/questions/11004374/watch-a-memory-range-in-gdb

An excerpt from above StackOverflow page to highlight the issue:

11

I am debugging a program in gdb and I want the program to stop when the memory region 0x08049000 to 0x0804a000 is accessed. When I try to set memory breakpoints manually, gdb does not seem to support more than two locations at a time.

(gdb) awatch *0x08049000
Hardware access (read/write) watchpoint 1: *0x08049000
(gdb) awatch *0x08049001
Hardware access (read/write) watchpoint 2: *0x08049001
(gdb) awatch *0x08049002
Hardware access (read/write) watchpoint 3: *0x08049002
(gdb) run
Starting program: /home/iblue/git/some-code/some-executable
Warning:
Could not insert hardware watchpoint 3.
Could not insert hardware breakpoints:
You may have requested too many hardware breakpoints/watchpoints.

There is already a question where this has been asked and the answer was, that it may be possible to do this with valgrind. Unfortunately the answer does not contain any examples or reference to the valgrind manual, so it was not very enlightning: How can gdb be used to watch for any changes in an entire region of memory?

So: How can I watch the whole memory region?
c
linux
debugging
gdb
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edited May 23 '17 at 12:32
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asked Jun 12 '12 at 20:30
iblue
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    Interesting fact: PowerPC has ranged breakpoints (but not watchpoints ?): stackoverflow.com/questions/13410941/… – Ciro Santilli 新疆再教育营六四事件法轮功郝海东 Jul 27 '15 at 16:09
    x86 supports small watch ranges up to 8 bytes: en.wikipedia.org/wiki/X86_debug_register – Ciro Santilli 新疆再教育营六四事件法轮功郝海东 Aug 12 '15 at 12:09

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2 Answers
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If you use GDB 7.4 together with Valgrind 3.7.0, then you have unlimited "emulated" hardware watchpoints.

Start your program under Valgrind, giving the arguments --vgdb=full --vgdb-error=0 then use GDB to connect to it (target remote | vgdb). Then you can e.g. watch or awatch or rwatch a memory range by doing rwatch (char[100]) *0x5180040

See the Valgrind user manual on gdb integration for more details
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edited Jun 17 '12 at 17:57
iblue
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answered Jun 13 '12 at 20:00
phd
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    4
    After I spent the better part of the day fiddling with mprotect and abusing SIGSEV handlers to break on memory access, I tried this. It works perfectly. You saved my day. Thank you! – iblue Jun 13 '12 at 23:00
    Yes, +1 also. I've been hunting for a feature like this for months. – Crashworks Jun 14 '12 at 0:10
    So, how does one determine the heap address for the process started by valgrind? I usually do this via /proc/[pid]/maps but when I start python via this valgrind command, the maps file doesn't have an entry identified by [heap] as I'm used to finding. – Andrew Falanga Feb 11 '16 at 19:59 

I realise that this answer was written a long time ago, but I am confused by this answer, because gdb can do that by itself, without valgrind's help. At least it can now. It will resort to single-step and check repeatedly. That's obviously really expensive (like valgrind is), so you would narrow it down to when the corruption is about to happen soon, then create the watchpoint, so you don't have it running slow the whole debug run. – doug65536 Dec 10 '20 at 3:40