[PATCH v2] x86/fault: Decode and print #PF oops in human readable form

[Sean Christopherson]

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.

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

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---

v2:
  - Explicitly call out protection keys violations
  - "Slightly" reword the changelog

 arch/x86/mm/fault.c | 42 +++++++++++-------------------------------
 1 file changed, 11 insertions(+), 31 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 2ff25ad33233..26feb8c525c1 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -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;
 
@@ -648,27 +633,22 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
 		 address < PAGE_SIZE ? "NULL pointer dereference" : "paging request",
 		 (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: 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");
+	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");
 
 	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
-- 
2.19.2