Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6_magic tests

From: John David Anglin
Date: Thu Feb 15 2024 - 13:56:51 EST


On 2024-02-15 1:17 p.m., Guenter Roeck wrote:
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
It depends on how syscall_exit is declared.    unwind.c lies the type of handle_interruption, etc:

        extern void * const handle_interruption;
        extern void * const ret_from_kernel_thread;
        extern void * const syscall_exit;
        extern void * const intr_return;
        extern void * const _switch_to_ret;
#ifdef CONFIG_IRQSTACKS
        extern void * const _call_on_stack;
#endif /* CONFIG_IRQSTACKS */

This should yield actual addresses.
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.
Looks like.

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.
This is Helge's code...  I'll let him fix it.

Dave

--
John David Anglin dave.anglin@xxxxxxxx