[PATCH] Wrong symbol resolved for RIP on OOPS/BUG

[Marek Majkowski <marek@cloudflare.com>]

"%pB" is intended for return addresses, and actually resolves the
address - 1.  So it should only be used for backtraces.  Plain
instruction addresses should use "%pS", which resolves the given
address.

show_regs was using "%pB" to resolve the RIP symbol. This resolved the
wrong symbol if the first instruction after a symbol created the
OOPS/BUG. For example:

0000000000000049 <before>:
  49:   90                      nop
  4a:   90                      nop
  4b:   90                      nop
  4c:   90                      nop
000000000000004d <suicide>:
  4d:   ff 14 25 00 00 00 00    callq  *0x0
  54:   c3                      retq

Will produce a message saying it's "before" that crashed, not "suicide".

This problem only happens when the crash occurs in the first instruction
after a symbol. Therefore it's unlikely to occur on kernels with frame
pointers (CONFIG_FRAME_POINTER=y).

Signed-off-by: Marek Majkowski <marek@cloudflare.com>

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index deb6421..4c90013 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -27,6 +27,12 @@ static int die_counter;
 
 void printk_address(unsigned long address, int reliable)
 {
+	pr_cont(" [<%p>] %s%pS\n",
+		(void *)address, reliable ? "" : "? ", (void *)address);
+}
+
+static void printk_trace_address(unsigned long address, int reliable)
+{
 	pr_cont(" [<%p>] %s%pB\n",
 		(void *)address, reliable ? "" : "? ", (void *)address);
 }
@@ -151,7 +157,7 @@ static void print_trace_address(void *data, unsigned long addr, int reliable)
 {
 	touch_nmi_watchdog();
 	printk(data);
-	printk_address(addr, reliable);
+	printk_trace_address(addr, reliable);
 }
 
 static const struct stacktrace_ops print_trace_ops = {
-- 
1.8.3.2