On 2/15/24 09:25, John David Anglin wrote:
[ ... ]
Source:
static bool pc_is_kernel_fn(unsigned long pc, void *fn)
{
return (unsigned long)dereference_kernel_function_descriptor(fn) == pc;
This looks wrong to me. Function descriptors should always be 8-byte aligned. I think this
routine should return false if fn isn't 8-byte aligned.
Below you state "Code entry points only need 4-byte alignment."
I think that contradicts each other. Also, the calling code is,
for example,
pc_is_kernel_fn(pc, syscall_exit)
I fail to see how this can be consolidated if it is ok
that syscall_exit is 4-byte aligned but, at the same time,
must be 8-byte aligned to be considered to be a kernel function.
In the above call, syscall_exit is treated as a function pointer. It points to an 8-byte aligned
function descriptor. The descriptor holds the actual address of the function. It only needs
4-byte alignment.
Descriptors need 8-byte alignment for efficiency on 64-bit parisc. The pc and gp are accessed
using ldd instructions.
Maybe code such as
pc_is_kernel_fn(pc, syscall_exit)
is wrong because syscall_exit doesn't point to a function descriptor
but to the actual address. The code and comments in arch/parisc/kernel/unwind.c
is for sure confusing because it talks about not using
dereference_kernel_function_descriptor() to keep things simple but then calls
dereference_kernel_function_descriptor() anyway. Maybe it should just be
if (pc == syscall_exit)
instead.
The entire code is really odd anyway.
ptr = dereference_kernel_function_descriptor(&handle_interruption);
if (pc_is_kernel_fn(pc, ptr)) {
and then pc_is_kernel_fn() dereferences it again. Weird.
It looks like commit 8e0ba125c2bf ("parisc/unwind: fix unwinder when
CONFIG_64BIT is enabled") might have messed this up. No idea how to fix
it properly, though.
Thanks,
Guenter
