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 10:42:29 -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:
> On 2024-02-15 12:06 p.m., Guenter Roeck wrote:
>> On 2/15/24 08:51, John David Anglin wrote:
>>> On 2024-02-15 10:44 a.m., Guenter Roeck wrote:
>>>> On 2/15/24 02:27, David Laight wrote:
>>>>> ...
>>>>>> It would be worthwhile tracking this down since there are
>>>>>> lots of unaligned data accesses (8-byte accesses on 4-byte aligned addresses)
>>>>>> when running the kernel in 64-bit mode.
>>>>>
>>>>> Hmmm....
>>>>> For performance reasons you really don't want any of them.
>>>>> The misaligned 64bit fields need an __attribute((aligned(4)) marker.
>>>>>
>>>>> If the checksum code can do them it really needs to detect
>>>>> and handle the misalignment.
>>>>>
>>>>> The misaligned trap handler probably ought to contain a
>>>>> warn_on_once() to dump stack on the first such error.
>>>>> They can then be fixed one at a time.
>>>>>
>>>>
>>>> Unaligned LDD at unwind_once+0x4a8/0x5e0
>>>>
>>>> Decoded:
>>>>
>>>> Unaligned LDD at unwind_once (arch/parisc/kernel/unwind.c:212 arch/parisc/kernel/unwind.c:243 arch/parisc/kernel/unwind.c:371 arch/parisc/kernel/unwind.c:445)
>>>>
>>>> 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.
> 

How about the patch below ?

Guenter

---
diff --git a/arch/parisc/kernel/unwind.c b/arch/parisc/kernel/unwind.c
index 27ae40a443b8..c2b9e23cbc0a 100644
--- a/arch/parisc/kernel/unwind.c
+++ b/arch/parisc/kernel/unwind.c
@@ -214,24 +214,14 @@ static bool pc_is_kernel_fn(unsigned long pc, void *fn)

  static int unwind_special(struct unwind_frame_info *info, unsigned long pc, int frame_size)
  {
-       /*
-        * We have to use void * instead of a function pointer, because
-        * function pointers aren't a pointer to the function on 64-bit.
-        * Make them const so the compiler knows they live in .text
-        * Note: We could use dereference_kernel_function_descriptor()
-        * instead but we want to keep it simple here.
-        */
-       extern void * const ret_from_kernel_thread;
-       extern void * const syscall_exit;
-       extern void * const intr_return;
-       extern void * const _switch_to_ret;
+       void (*ret_from_kernel_thread)(void);
+       void (*syscall_exit)(void);
+       void (*intr_return)(void);
+       void (*_switch_to_ret)(void);
  #ifdef CONFIG_IRQSTACKS
-       extern void * const _call_on_stack;
+       void (*_call_on_stack)(void);
  #endif /* CONFIG_IRQSTACKS */
-       void *ptr;
-
-       ptr = dereference_kernel_function_descriptor(&handle_interruption);
-       if (pc_is_kernel_fn(pc, ptr)) {
+       if (pc_is_kernel_fn(pc, handle_interruption)) {
                 struct pt_regs *regs = (struct pt_regs *)(info->sp - frame_size - PT_SZ_ALGN);
                 dbg("Unwinding through handle_interruption()\n");
                 info->prev_sp = regs->gr[30];