Re: [tip: x86/fred] x86/ptrace: Cleanup the definition of the pt_regs structure

[H. Peter Anvin]

On February 6, 2024 11:04:13 AM PST, Xin Li <xin@xxxxxxxxx> wrote:
>On 2/3/2024 3:52 PM, H. Peter Anvin wrote:
>> On January 31, 2024 1:14:52 PM PST, tip-bot2 for Xin Li <tip-bot2@xxxxxxxxxxxxx> wrote:
>>> The following commit has been merged into the x86/fred branch of tip:
>>> 
>>> Commit-ID:     ee63291aa8287cb7ded767d340155fe8681fc075
>>> Gitweb:        https://git.kernel.org/tip/ee63291aa8287cb7ded767d340155fe8681fc075
>>> Author:        Xin Li <xin3.li@xxxxxxxxx>
>>> AuthorDate:    Tue, 05 Dec 2023 02:50:02 -08:00
>>> Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
>>> CommitterDate: Wed, 31 Jan 2024 22:01:13 +01:00
>>> 
>>> x86/ptrace: Cleanup the definition of the pt_regs structure
>>> 
>>> struct pt_regs is hard to read because the member or section related
>>> comments are not aligned with the members.
>>> 
>>> The 'cs' and 'ss' members of pt_regs are type of 'unsigned long' while
>>> in reality they are only 16-bit wide. This works so far as the
>>> remaining space is unused, but FRED will use the remaining bits for
>>> other purposes.
>>> 
>>> To prepare for FRED:
>>> 
>>>   - Cleanup the formatting
>>>   - Convert 'cs' and 'ss' to u16 and embed them into an union
>>>     with a u64
>>>   - Fixup the related printk() format strings
>>> 
>>> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> Originally-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
>>> Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
>>> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
>>> Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
>>> Link: https://lore.kernel.org/r/20231205105030.8698-14-xin3.li@xxxxxxxxx
>
>[...]
>
>>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>>> index 33b2687..0f78b58 100644
>>> --- a/arch/x86/kernel/process_64.c
>>> +++ b/arch/x86/kernel/process_64.c
>>> @@ -117,7 +117,7 @@ void __show_regs(struct pt_regs *regs, enum show_regs_mode mode,
>>> 
>>> 	printk("%sFS:  %016lx(%04x) GS:%016lx(%04x) knlGS:%016lx\n",
>>> 	       log_lvl, fs, fsindex, gs, gsindex, shadowgs);
>>> -	printk("%sCS:  %04lx DS: %04x ES: %04x CR0: %016lx\n",
>>> +	printk("%sCS:  %04x DS: %04x ES: %04x CR0: %016lx\n",
>>> 		log_lvl, regs->cs, ds, es, cr0);
>>> 	printk("%sCR2: %016lx CR3: %016lx CR4: %016lx\n",
>>> 		log_lvl, cr2, cr3, cr4);
>> 
>> Incidentally, the comment about callee-saved registers is long since both obsolete and is now outright wrong.
>> 
>> The next version of gcc (14 I think) will have an attribute to turn off saving registers which we can use for top-level C functions.
>> 
>
>Forgive my ignorance, do we have an official definition for "top-level C functions"?
>
>Thanks!
>    Xin
>

(Adding H.J., who did the gcc implementation of __attribute__((no_callee_saved_registers))).

The top level C functions are the ones whose stack frame are immediately below the exception/syscall frame, i.e. the C function called from the entry assembly code and functions tailcalled from those (unless they set up a stack frame for things like memory structures passed to the called function.)

Note that the implementation should properly handle the case when calling these functions from C (accidentally, or because it is a rare case that can be validly pessimized.)