Hi guys, any update on this? Just to be sure, I tried to the Linux kernel 4.19.16 in the same scenario and I got the same result. I built the kernel with buildroot and I launched QEMU with:
qemu-system-i386 -kernel bzImage -S -s -append 'nokaslr' I know it needs an initrd and a hdd img in order to boot a full system, but for me it was enough to break on start_kernel and then trying to do `stepi`. Exactly like with the other project, with the gdb version `Ubuntu 8.1-0ubuntu3` it worked perfectly, while with gdb `Ubuntu 8.1-0ubuntu3.1` I got the same problem: (gdb) b start_kernel warning: Breakpoint address adjusted from 0xc17257cd to 0xffffffffc17257cd. Breakpoint 1 at 0xc17257cd (gdb) c Continuing. Program received signal SIGTRAP, Trace/breakpoint trap. 0xc17257cd in start_kernel () (gdb) si 0xc17257cd in start_kernel () (gdb) si 0xc17257cd in start_kernel () (gdb) si 0xc17257cd in start_kernel () (gdb) si Therefore, as expected, the bug affects _definitively_ any kind of 32-bit code when remote debugging is used and the client is 64-bit. I also checked if the latest non-Ubuntu gdb is affected by this issue and it's not. In conclusion, I believe that the following patch introduced the regression: http://launchpadlibrarian.net/431301516/gdb_8.1-0ubuntu3_8.1-0ubuntu3.1.diff.gz And that the bug needs to get some attention. After all, people _cannot_ debug a 32-bit linux kernel running on a VM anymore, if they're using Ubuntu. @Manoj could you please comment? Thanks -- 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: New 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
