linux-kernel - Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for ip_fast_csum and csum_ipv6

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Date: Thu, 15 Feb 2024 10:17:09 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: John David Anglin <dave.anglin@...l.net>,
 David Laight <David.Laight@...LAB.COM>,
 Charlie Jenkins <charlie@...osinc.com>, Palmer Dabbelt <palmer@...belt.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Helge Deller <deller@....de>,
 "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 Parisc List <linux-parisc@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 2/2] lib: checksum: Use aligned accesses for
 ip_fast_csum and csum_ipv6_magic tests

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