gdb (v 10.2) cannot access memory at ffffc9xxxxxxxxxx in x86_64 (vmalloc range)

[SanjibDas]

My gdb (GNU gdb (GDB) 10.2) backtrace fails to access kernel memory in vmalloc range. This is with a kernel crash coredump on x86_64 platform.

In my understanding 0xffffc90000000000 is the vmalloc start range with 48-bit virtual addressing system. https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt

ffffc90000000000 | -55 TB | ffffe8ffffffffff | 32 TB | vmalloc/ioremap space (vmalloc_base)

(gdb) bt
#0 0xffffffff817807ab in neigh_get_next (n=0xffff888129ff0a00, pos=0x0 <fixed_percpu_data>,
seq=<optimized out>, seq=<optimized out>) at net/core/neighbour.c:3220
Backtrace stopped: Cannot access memory at address 0xffffc90000c77dc0

However linux crash utility helped me to dump the complete backtrace. All stack frame addresses are in 0xffffc9xxxxxxxxxx address range.

crash>bt
#6 [ffffc90000c77dc0] neigh_seq_next at ffffffff8178096d
#7 [ffffc90000c77de0] seq_read_iter at ffffffff812302c4
#8 [ffffc90000c77e38] seq_read at ffffffff8123054e
#9 [ffffc90000c77eb8] proc_reg_read at ffffffff812806b6
#10 [ffffc90000c77ed0] vfs_read at ffffffff812046c0
#11 [ffffc90000c77f08] ksys_read at ffffffff81204a7a
#12 [ffffc90000c77f40] do_syscall_64 at ffffffff81a17bbd
#13 [ffffc90000c77f50] entry_SYSCALL_64_after_hwframe at ffffffff81c00099

To debug further, I dumped all sections from vmlinux (gdb> maintenance info sections, info files). Address range 0xffffc9xxxxxxxxxx is missing there.

gdb usage has become very limited in this case. I am yet to figure out the missing piece. If anyone has encountered similar issue in the past & have any pointer, please share with me.

Thanks in advance.

[Diego_INTEL]

Hello @SanjibDas,

Thank you for contacting Intel Embedded Community.

Unfortunately I have not found much details internally, but maybe you can check this:

https://patchwork.kernel.org/project/linux-mediatek/patch/20230725093458.30064-9-Kuan-Ying.Lee@mediatek.com/

https://www.intel.com/content/www/us/en/developer/tools/oneapi/distribution-for-gdb.html

Best regards,

@Diego_INTEL 

[SanjibDas]

Appreciate your reply/comments.

I will explore those links.

Thanks,

Sanjib