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x86: Avoid pr_cont() in show_opcodes()

If concurrent printk() messages are emitted, then pr_cont() is making it
extremly hard to decode which part of the output belongs to what. See the
convoluted example at:

  https://syzkaller.appspot.com/text?tag=CrashReport&x=139d342c400000

Avoid this by using a proper prefix for each line and by using %ph format
in show_opcodes() which emits the 'Code:' line in one go.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: joe@perches.com
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@suse.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/1532009278-5953-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
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Villemoes authored and Thomas Gleixner committed Jul 19, 2018
1 parent 9d3cce1 commit 8e974b3b8eddd42a8c43a18e8ea66a3e7b803a0d
Showing with 9 additions and 19 deletions.
  1. +9 −19 arch/x86/kernel/dumpstack.c
@@ -22,8 +22,6 @@
#include <asm/stacktrace.h>
#include <asm/unwind.h>

#define OPCODE_BUFSIZE 64

int panic_on_unrecovered_nmi;
int panic_on_io_nmi;
static int die_counter;
@@ -93,26 +91,18 @@ static void printk_stack_address(unsigned long address, int reliable,
*/
void show_opcodes(u8 *rip, const char *loglvl)
{
unsigned int code_prologue = OPCODE_BUFSIZE * 2 / 3;
#define PROLOGUE_SIZE 42
#define EPILOGUE_SIZE 21
#define OPCODE_BUFSIZE (PROLOGUE_SIZE + 1 + EPILOGUE_SIZE)
u8 opcodes[OPCODE_BUFSIZE];
u8 *ip;
int i;

printk("%sCode: ", loglvl);

ip = (u8 *)rip - code_prologue;
if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
pr_cont("Bad RIP value.\n");
return;
}

for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
if (ip == rip)
pr_cont("<%02x> ", opcodes[i]);
else
pr_cont("%02x ", opcodes[i]);
if (probe_kernel_read(opcodes, rip - PROLOGUE_SIZE, OPCODE_BUFSIZE)) {
printk("%sCode: Bad RIP value.\n", loglvl);
} else {
printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %"
__stringify(EPILOGUE_SIZE) "ph\n", loglvl, opcodes,
opcodes[PROLOGUE_SIZE], opcodes + PROLOGUE_SIZE + 1);
}
pr_cont("\n");
}

void show_ip(struct pt_regs *regs, const char *loglvl)

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