Linux Kernel Module Cheat

3.7. GDB call

GDB can call functions as explained at: https://stackoverflow.com/questions/1354731/how-to-evaluate-functions-in-gdb

However this is failing for us:

some symbols are not visible to call even though b sees them
for those that are, call fails with an E14 error

E.g.: if we break on __x64_sys_write on count.sh:

>>> call printk(0, "asdf")
Could not fetch register "orig_rax"; remote failure reply 'E14'
>>> b printk
Breakpoint 2 at 0xffffffff81091bca: file kernel/printk/printk.c, line 1824.
>>> call fdget_pos(fd)
No symbol "fdget_pos" in current context.
>>> b fdget_pos
Breakpoint 3 at 0xffffffff811615e3: fdget_pos. (9 locations)
>>>

even though fdget_pos is the first thing __x64_sys_write does:

581 SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
582         size_t, count)
583 {
584     struct fd f = fdget_pos(fd);

I also noticed that I get the same error:

Could not fetch register "orig_rax"; remote failure reply 'E14'

when trying to use:

fin

on many (all?) functions.