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

[Xin Li]

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