linux-kernel - [PATCH v4 33/57] x86/dumpstack: simplify in_exception

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Message-Id: <8e382a3d77e57296a892dd9108d8268974175896.1471525031.git.jpoimboe@redhat.com>
Date:	Thu, 18 Aug 2016 08:06:13 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>,
	Nilay Vaish <nilayvaish@...il.com>
Subject: [PATCH v4 33/57] x86/dumpstack: simplify in_exception_stack()

in_exception_stack() does some bad, bad things just so the unwinder can
print different values for different areas of the debug exception stack.

There's no need to clarify where exactly on the stack it is.  Just print
"#DB" and be done with it.

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/kernel/dumpstack_64.c | 89 ++++++++++++------------------------------
 1 file changed, 26 insertions(+), 63 deletions(-)

diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index f1b843a..69f6ba2 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -16,83 +16,46 @@
 
 #include <asm/stacktrace.h>
 
+static char *exception_stack_names[N_EXCEPTION_STACKS] = {
+		[ DOUBLEFAULT_STACK-1	]	= "#DF",
+		[ NMI_STACK-1		]	= "NMI",
+		[ DEBUG_STACK-1		]	= "#DB",
+		[ MCE_STACK-1		]	= "#MC",
+};
 
-#define N_EXCEPTION_STACKS_END \
-		(N_EXCEPTION_STACKS + DEBUG_STKSZ/EXCEPTION_STKSZ - 2)
-
-static char x86_stack_ids[][8] = {
-		[ DEBUG_STACK-1			]	= "#DB",
-		[ NMI_STACK-1			]	= "NMI",
-		[ DOUBLEFAULT_STACK-1		]	= "#DF",
-		[ MCE_STACK-1			]	= "#MC",
-#if DEBUG_STKSZ > EXCEPTION_STKSZ
-		[ N_EXCEPTION_STACKS ...
-		  N_EXCEPTION_STACKS_END	]	= "#DB[?]"
-#endif
+static unsigned long exception_stack_sizes[N_EXCEPTION_STACKS] = {
+	[0 ... N_EXCEPTION_STACKS - 1]		= EXCEPTION_STKSZ,
+	[DEBUG_STACK - 1]			= DEBUG_STKSZ
 };
 
 static unsigned long *in_exception_stack(unsigned long stack, unsigned *usedp,
 					 char **idp)
 {
+	unsigned long begin, end;
 	unsigned k;
 
-	/*
-	 * Iterate over all exception stacks, and figure out whether
-	 * 'stack' is in one of them:
-	 */
+	BUILD_BUG_ON(N_EXCEPTION_STACKS != 4);
+
 	for (k = 0; k < N_EXCEPTION_STACKS; k++) {
-		unsigned long end = raw_cpu_ptr(&orig_ist)->ist[k];
-		/*
-		 * Is 'stack' above this exception frame's end?
-		 * If yes then skip to the next frame.
-		 */
-		if (stack >= end)
+		end   = raw_cpu_ptr(&orig_ist)->ist[k];
+		begin = end - exception_stack_sizes[k];
+
+		if (stack < begin || stack >= end)
 			continue;
+
 		/*
-		 * Is 'stack' above this exception frame's start address?
-		 * If yes then we found the right frame.
-		 */
-		if (stack >= end - EXCEPTION_STKSZ) {
-			/*
-			 * Make sure we only iterate through an exception
-			 * stack once. If it comes up for the second time
-			 * then there's something wrong going on - just
-			 * break out and return NULL:
-			 */
-			if (*usedp & (1U << k))
-				break;
-			*usedp |= 1U << k;
-			*idp = x86_stack_ids[k];
-			return (unsigned long *)end;
-		}
-		/*
-		 * If this is a debug stack, and if it has a larger size than
-		 * the usual exception stacks, then 'stack' might still
-		 * be within the lower portion of the debug stack:
+		 * Make sure we only iterate through an exception stack once.
+		 * If it comes up for the second time then there's something
+		 * wrong going on - just break and return NULL:
 		 */
-#if DEBUG_STKSZ > EXCEPTION_STKSZ
-		if (k == DEBUG_STACK - 1 && stack >= end - DEBUG_STKSZ) {
-			unsigned j = N_EXCEPTION_STACKS - 1;
+		if (*usedp & (1U << k))
+			break;
+		*usedp |= 1U << k;
 
-			/*
-			 * Black magic. A large debug stack is composed of
-			 * multiple exception stack entries, which we
-			 * iterate through now. Dont look:
-			 */
-			do {
-				++j;
-				end -= EXCEPTION_STKSZ;
-				x86_stack_ids[j][4] = '1' +
-						(j - N_EXCEPTION_STACKS);
-			} while (stack < end - EXCEPTION_STKSZ);
-			if (*usedp & (1U << j))
-				break;
-			*usedp |= 1U << j;
-			*idp = x86_stack_ids[j];
-			return (unsigned long *)end;
-		}
-#endif
+		*idp = exception_stack_names[k];
+		return (unsigned long *)end;
 	}
+
 	return NULL;
 }
 
-- 
2.7.4