Ignore my last comment - I took another look and it appears to be generic. -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to gdb in Ubuntu. https://bugs.launchpad.net/bugs/1846557
Title: Unable to debug any kernel on i386 qemu machine Status in gdb package in Ubuntu: Fix Released Bug description: Hi, On my x86_64 machine [running Ubuntu 18.04.3 LTS] with gdb version 'Ubuntu 8.1-0ubuntu3' I could happily debug any kernel running on a i386 qemu VM (qemu-system-i386) by just doing the following: > target remote localhost:1234 > b term.c:694 and then, when the breakpoint was hit I used to observe output like: > Breakpoint 1, term_action_use_alt_buffer (t=0xc017514c <first_instance>, use_alt_buffer=true) > at /home/vlad/dev/tilck/kernel/char/tty/term.c:694 And then I was able to do `s`, `si` or `c`, exactly like with regular user applications. With the newest update of gdb, version 'Ubuntu 8.1-0ubuntu3.1', instead, something is broken. By doing the same things I observe: > (gdb) b term.c:693 > warning: Breakpoint address adjusted from 0xc01158fe to 0xffffffffc01158fe. Which seems (and actually is) a bad sign, for what comes later. [why do you need to change the address? why do you want to extend it to 64-bit for a 32-bit machine?? mmm..] GDB detects the breakpoint, but in a weird way: Program received signal SIGTRAP, Trace/breakpoint trap. term_action_use_alt_buffer (t=0xc017514c <first_instance>, use_alt_buffer=true) At this point, I'm able to read the memory and the variables BUT, I cannot continue the execution, NOR doing any kind of step. The commands apparently don't get delivered to the remote side (QEMU), or they get delivered in a wrong way somehow. Example output: (gdb) b 709 warning: Breakpoint address adjusted from 0xc0115a45 to 0xffffffffc0115a45. Breakpoint 2 at 0xc0115a45: file /home/vlad/dev/tilck/kernel/char/tty/term.c, line 709. (gdb) c Continuing. Program received signal SIGTRAP, Trace/breakpoint trap. term_action_use_alt_buffer (t=0xc017514c <first_instance>, use_alt_buffer=true) at /home/vlad/dev/tilck/kernel/char/tty/term.c:693 693 t->alt_buf = kmalloc(sizeof(u16) * t->rows * t->cols); (gdb) c Continuing. Program received signal SIGTRAP, Trace/breakpoint trap. term_action_use_alt_buffer (t=0xc017514c <first_instance>, use_alt_buffer=true) at /home/vlad/dev/tilck/kernel/char/tty/term.c:693 693 t->alt_buf = kmalloc(sizeof(u16) * t->rows * t->cols); (gdb) c Continuing. As you see, the whole QEMU VM is stuck until I quit GDB. Note: I downgraded exclusively GDB back to version 'Ubuntu 8.1-0ubuntu3' in order to check if the problem would be fixed and it is. I'm sure the problem has been introduced in this specific version 'Ubuntu 8.1-0ubuntu3.1' and it's *not* related with QEMU *nor* with the kernel that is being debugged. It's totally independent from that. Final remark: note that I'm running gdb on x86_64 machine, while I'm debugging a kernel running on a i386 (virtual) machine. I believe that the cross-arch scenario almost certainly has something to do with the bug, as it happened in the past on both sides (qemu and gdb). Thanks a lot, Vlad To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gdb/+bug/1846557/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
