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Date: Tue, 06 Feb 2024 12:45:03 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
To: Xin Li <xin@zytor.com>, linux-kernel@vger.kernel.org,
        tip-bot2 for Xin Li <tip-bot2@linutronix.de>,
        linux-tip-commits@vger.kernel.org, "H.J. Lu" <hjl.tools@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>,
        "Borislav Petkov (AMD)" <bp@alien8.de>,
        Shan Kang <shan.kang@intel.com>, x86@kernel.org
Subject: =?US-ASCII?Q?Re=3A_=5Btip=3A_x86/fred=5D_x86/ptrace=3A_Cleanup?= =?US-ASCII?Q?_the_definition_of_the_pt=5Fregs_structure?=
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References: <20231205105030.8698-14-xin3.li@intel.com> <170673569232.398.15041548048531772130.tip-bot2@tip-bot2> <2A59D51B-0AA0-4DFC-9957-67CC0C9E37B3@zytor.com> <8f260a93-08f3-48af-81e5-8ee53246e262@zytor.com>
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On February 6, 2024 11:04:13 AM PST, Xin Li <xin@zytor=2Ecom> wrote:
>On 2/3/2024 3:52 PM, H=2E Peter Anvin wrote:
>> On January 31, 2024 1:14:52 PM PST, tip-bot2 for Xin Li <tip-bot2@linut=
ronix=2Ede> wrote:
>>> The following commit has been merged into the x86/fred branch of tip:
>>>=20
>>> Commit-ID:     ee63291aa8287cb7ded767d340155fe8681fc075
>>> Gitweb:        https://git=2Ekernel=2Eorg/tip/ee63291aa8287cb7ded767d3=
40155fe8681fc075
>>> Author:        Xin Li <xin3=2Eli@intel=2Ecom>
>>> AuthorDate:    Tue, 05 Dec 2023 02:50:02 -08:00
>>> Committer:     Borislav Petkov (AMD) <bp@alien8=2Ede>
>>> CommitterDate: Wed, 31 Jan 2024 22:01:13 +01:00
>>>=20
>>> x86/ptrace: Cleanup the definition of the pt_regs structure
>>>=20
>>> struct pt_regs is hard to read because the member or section related
>>> comments are not aligned with the members=2E
>>>=20
>>> The 'cs' and 'ss' members of pt_regs are type of 'unsigned long' while
>>> in reality they are only 16-bit wide=2E This works so far as the
>>> remaining space is unused, but FRED will use the remaining bits for
>>> other purposes=2E
>>>=20
>>> To prepare for FRED:
>>>=20
>>>   - Cleanup the formatting
>>>   - Convert 'cs' and 'ss' to u16 and embed them into an union
>>>     with a u64
>>>   - Fixup the related printk() format strings
>>>=20
>>> Suggested-by: Thomas Gleixner <tglx@linutronix=2Ede>
>>> Originally-by: H=2E Peter Anvin (Intel) <hpa@zytor=2Ecom>
>>> Signed-off-by: Xin Li <xin3=2Eli@intel=2Ecom>
>>> Signed-off-by: Thomas Gleixner <tglx@linutronix=2Ede>
>>> Signed-off-by: Borislav Petkov (AMD) <bp@alien8=2Ede>
>>> Tested-by: Shan Kang <shan=2Ekang@intel=2Ecom>
>>> Link: https://lore=2Ekernel=2Eorg/r/20231205105030=2E8698-14-xin3=2Eli=
@intel=2Ecom
>
>[=2E=2E=2E]
>
>>> diff --git a/arch/x86/kernel/process_64=2Ec b/arch/x86/kernel/process_=
64=2Ec
>>> index 33b2687=2E=2E0f78b58 100644
>>> --- a/arch/x86/kernel/process_64=2Ec
>>> +++ b/arch/x86/kernel/process_64=2Ec
>>> @@ -117,7 +117,7 @@ void __show_regs(struct pt_regs *regs, enum show_r=
egs_mode mode,
>>>=20
>>> 	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);
>>=20
>> Incidentally, the comment about callee-saved registers is long since bo=
th obsolete and is now outright wrong=2E
>>=20
>> 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=2E
>>=20
>
>Forgive my ignorance, do we have an official definition for "top-level C =
functions"?
>
>Thanks!
>    Xin
>

(Adding H=2EJ=2E, who did the gcc implementation of __attribute__((no_call=
ee_saved_registers)))=2E

The top level C functions are the ones whose stack frame are immediately b=
elow the exception/syscall frame, i=2Ee=2E the C function called from the e=
ntry assembly code and functions tailcalled from those (unless they set up =
a stack frame for things like memory structures passed to the called functi=
on=2E)

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=2E)=20