x86/fault: Clean up the page fault oops decoder a bit

 - Make the oops messages a bit less scary (don't mention 'HW errors')

 - Turn 'PROT USER' (which is visually easily confused with PROT_USER)
   into individual bit descriptors: "[PROT] [USER]".
   This also makes "[normal kernel read fault]" more apparent.

 - De-abbreviate variables to make the code easier to read

 - Use vertical alignment where appropriate.

 - Add comment about string size limits and the helper function.

 - Remove unnecessary line breaks.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
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>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/fault.c

@@ -603,10 +603,13 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
 		 name, index, addr, (desc.limit0 | (desc.limit1 << 16)));
 }
 
-static void errstr(unsigned long ec, char *buf, unsigned long mask,
-		   const char *txt)
+/*
+ * 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 (ec & mask) {
+	if (error_code & mask) {
 		if (buf[0])
 			strcat(buf, " ");
 		strcat(buf, txt);
@@ -614,10 +617,9 @@ static void errstr(unsigned long ec, char *buf, unsigned long mask,
 }
 
 static void
-show_fault_oops(struct pt_regs *regs, unsigned long error_code,
-		unsigned long address)
+show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long address)
 {
-	char errtxt[64];
+	char err_txt[64];
 
 	if (!oops_may_print())
 		return;
@@ -646,15 +648,21 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code,
 		 address < PAGE_SIZE ? "NULL pointer dereference" : "paging request",
 		 (void *)address);
 
-	errtxt[0] = 0;
-	errstr(error_code, errtxt, X86_PF_PROT, "PROT");
-	errstr(error_code, errtxt, X86_PF_WRITE, "WRITE");
-	errstr(error_code, errtxt, X86_PF_USER, "USER");
-	errstr(error_code, errtxt, X86_PF_RSVD, "RSVD");
-	errstr(error_code, errtxt, X86_PF_INSTR, "INSTR");
-	errstr(error_code, errtxt, X86_PF_PK, "PK");
-	pr_alert("HW error: %s\n", error_code ? errtxt :
-		 "normal kernel read fault");
+	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]");
+
 	if (!(error_code & X86_PF_USER) && user_mode(regs)) {
 		struct desc_ptr idt, gdt;
 		u16 ldtr, tr;