linux-kernel - Re: [PATCH] x86/fault: Print "SUPERVISOR" and "READ" when decoding #PF oops

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Date:   Wed, 5 Dec 2018 11:27:13 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...riel.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] x86/fault: Print "SUPERVISOR" and "READ" when decoding
 #PF oops

On 12/5/18 8:36 AM, Sean Christopherson wrote:
> ...instead of manually handling the case where error_code=0, e.g. to
> display "[SUPERVISOR] [READ]" instead of "normal kernel read fault".
> 
> This makes the zero case consistent with all other messages and also
> provides additional information for other error code combinations,
> e.g. error_code==1 will display "[PROT] [SUPERVISOR] [READ]" instead
> of simply "[PROT]".
> 
> Print unique names for the negative cases as opposed to e.g. "[!USER]"
> to avoid mixups due to users missing a single "!" character, and to be
> more concise for the !INSTR && !WRITE case.
> 
> Print "SUPERVISOR" in favor of "KERNEL" to reduce the likelihood that
> the message is misinterpreted as a generic kernel/software error and
> to be consistent with the SDM's nomenclature.
> 
> An alternative to passing a negated error code to err_str_append() would
> be to expand err_str_append() to take a second string for the negative
> test, but that approach complicates handling the "[READ]" case, which
> looks for !INSTR && !WRITE, e.g. it would require an extra call to
> err_str_append() and logic in err_str_append() to allow null messages
> for both the positive and negative tests.  Printing "[INSTR] [READ]"
> wouldn't be the end of the world, but a little bit of trickery in the
> kernel is a relatively small price to pay in exchange for the ability
> to unequivocally know the access type by reading a single word.
> 
> Now that all components of the message use the [<code>] format,
> explicitly state that it's the error *code* that's being printed and
> group the err_str_append() calls by type so that the resulting print
> messages are consistent, e.g. the deciphered codes will always be:
> 
>     [PROT] [USER|SUPERVISOR] [WRITE|INSTR|READ] [RSDV] [PK]

                                                   RSVD
but it's correct in the patch.
BTW, what does PK mean?

thanks.

> 
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/mm/fault.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 2ff25ad33233..0b4ce5d2b461 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -609,7 +609,7 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
>   */
>  static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt)
>  {
> -	if (error_code & mask) {
> +	if ((error_code & mask) == mask) {
>  		if (buf[0])
>  			strcat(buf, " ");
>  		strcat(buf, txt);
> @@ -655,13 +655,16 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
>  	 * zero delimiter must fit into err_txt[].
>  	 */
>  	err_str_append(error_code, err_txt, X86_PF_PROT,  "[PROT]" );
> -	err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]");
>  	err_str_append(error_code, err_txt, X86_PF_USER,  "[USER]" );
> -	err_str_append(error_code, err_txt, X86_PF_RSVD,  "[RSVD]" );
> +	err_str_append(~error_code, err_txt, X86_PF_USER, "[SUPERVISOR]");
> +	err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]");
>  	err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]");
> +	err_str_append(~error_code, err_txt, X86_PF_WRITE | X86_PF_INSTR,
> +							  "[READ]");
> +	err_str_append(error_code, err_txt, X86_PF_RSVD,  "[RSVD]" );
>  	err_str_append(error_code, err_txt, X86_PF_PK,    "[PK]"   );
>  
> -	pr_alert("#PF error: %s\n", error_code ? err_txt : "[normal kernel read fault]");
> +	pr_alert("#PF error code: %s\n", err_txt);
>  
>  	if (!(error_code & X86_PF_USER) && user_mode(regs)) {
>  		struct desc_ptr idt, gdt;
> 


-- 
~Randy