I have NetBSD-current running on an RPi 3. When I attach to a program (which is waiting for the debugger in a loop) with gdb and try to step, or perform any activity that involves writing to the program text such as inserting a breakpoint, I see that the ptrace (PT_IO,...) call fails with errno EFAULT. EFAULT is not among the documented errnos set by ptrace.
I spent some time trying to find the root cause of this failure. The EFAULT is generated from a call to copyin() in src/sys/kern/subr_copy.c:copyin_vmspace(). I suspect that the call is failing because the page protection bits are not being set correctly on attach
on the RPi.
I can reproduce this on NetBSD-current [8.99.3] on evbarm (RPi).
I dont have a spare machine/SD-card lying around to test this on -release. FWIW I cannot reproduce this with either -current or -release on amd64 though. Also FWIW, continuing without setting any breakpoints seems to work fine.
I am pasting a session transcript below for reference.
Script started on Sat Nov 4 00:35:43 2017
$ uname -a
NetBSD XXX 8.99.3 NetBSD 8.99.3 (RPI2) #0: Sun Oct 1 10:45:49 UTC 2017 mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/evbarm/compile/RPI2 evbarm
$ ./loop &
[1] 630
$ gdb -pid=630
GNU gdb (GDB) 7.12
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Attaching to process 630
Reading symbols from /usr/home/xxx/C/loop...done.
Reading symbols from /usr/lib/libc.so.12...(no debugging symbols found)...done.
Reading symbols from /usr/libexec/ld.elf_so...(no debugging symbols found)...done.
[Switching to LWP 1]
0x0001073c in loop () at loop.c:8
8 while (x) {
(gdb) n
0x0001073c in loop () at loop.c:8
8 while (x) {
Warning:
Cannot insert breakpoint 0.
Cannot access memory at address 0x10740
(gdb)
0x0001073c in loop () at loop.c:8
8 while (x) {
Warning:
Cannot insert breakpoint 0.
Cannot access memory at address 0x10740
(gdb) q
A debugging session is active.
Inferior 1 [LWP 0] will be detached.
Quit anyway? (y or n) y
Detaching from program: /usr/home/xxx/C/loop, process 630
$ exit
Script done on Sat Nov 4 00:36:25 2017
Thanks and regards,