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Commit 18ea35c5 authored by Sean Christopherson's avatar Sean Christopherson Committed by Ingo Molnar
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x86/fault: Decode and print #PF oops in human readable form 

Linus pointed out that deciphering the raw #PF error code and printing
a more human readable message are two different things, and also that
printing the negative cases is mostly just noise[1].  For example, the
USER bit doesn't mean the fault originated in user code and stating
that an oops wasn't due to a protection keys violation isn't interesting
since an oops on a keys violation is a one-in-a-million scenario.

Remove the per-bit decoding of the error code and instead print:
  - the raw error code
  - why the fault occurred
  - the effective privilege level of the access
  - the type of access
  - whether the fault originated in user code or kernel code

This provides the user with the information needed to triage 99.9% of
oopses without polluting the log with useless information or conflating
the error_code with the CPL.

Sample output:

    BUG: kernel NULL pointer dereference, address = 0000000000000008
    #PF: supervisor-privileged instruction fetch from kernel code
    #PF: error_code(0x0010) - not-present page

    BUG: unable to handle page fault for address = ffffbeef00000000
    #PF: supervisor-privileged instruction fetch from kernel code
    #PF: error_code(0x0010) - not-present page

    BUG: unable to handle page fault for address = ffffc90000230000
    #PF: supervisor-privileged write access from kernel code
    #PF: error_code(0x000b) - reserved bit violation

[1] https://lkml.kernel.org/r/CAHk-=whk_fsnxVMvF1T2fFCaP2WrvSybABrLQCWLJyCvHw6NKA@mail.gmail.com



Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20181221213657.27628-3-sean.j.christopherson@intel.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent f28b11a2

arch/x86/mm/fault.c

+11 −31
Original line number Diff line number Diff line
@@ -603,24 +603,9 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index) 
		 name, index, addr, (desc.limit0 | (desc.limit1 << 16)));

}



/*

 * This helper function transforms the #PF error_code bits into

 * "[PROT] [USER]" type of descriptive, almost human-readable error strings:

 */

static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt)

{

	if (error_code & mask) {

		if (buf[0])

			strcat(buf, " ");

		strcat(buf, txt);

	}

}



static void

show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long address)

{

	char err_txt[64];



	if (!oops_may_print())

		return;
@@ -651,27 +636,22 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad 
		pr_alert("BUG: unable to handle page fault for address = %px\n",

			(void *)address);



	err_txt[0] = 0;



	/*

	 * Note: length of these appended strings including the separation space and the

	 * 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_INSTR, "[INSTR]");

	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: %s-privileged %s from %s code\n",

		 (error_code & X86_PF_USER)  ? "user" : "supervisor",

		 (error_code & X86_PF_INSTR) ? "instruction fetch" :

		 (error_code & X86_PF_WRITE) ? "write access" :

					       "read access",

			     user_mode(regs) ? "user" : "kernel");

	pr_alert("#PF: error_code(0x%04lx) - %s\n", error_code,

		 !(error_code & X86_PF_PROT) ? "not-present page" :

		 (error_code & X86_PF_RSVD)  ? "reserved bit violation" :

		 (error_code & X86_PF_PK)    ? "protection keys violation" :

					       "permissions violation");



	if (!(error_code & X86_PF_USER) && user_mode(regs)) {

		struct desc_ptr idt, gdt;

		u16 ldtr, tr;



		pr_alert("This was a system access from user code\n");



		/*

		 * This can happen for quite a few reasons.  The more obvious

		 * ones are faults accessing the GDT, or LDT.  Perhaps