linux-kernel - [PATCH v3 2/3] powerpc/process: fix interleaved output in show_user

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Message-Id: <8f4de3aa04f00d67fdb5aaf90f41242197310d4d.1536327756.git.christophe.leroy@c-s.fr>
Date:   Fri,  7 Sep 2018 13:47:31 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, muriloo@...ux.ibm.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v3 2/3] powerpc/process: fix interleaved output in
 show_user_instructions()

When two processes crash at the same time, we sometimes encounter
interleaving in the middle of a line:

[    4.365317] init[1]: segfault (11) at 0 nip 0 lr 0 code 1
[    4.370452] init[1]: code: XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
[    4.372042] init[74]: segfault (11) at 10a74 nip 1000c198 lr 100078c8 code 1 in sh[10000000+14000]
[    4.386829] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
[    4.391542] init[1]: code: XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
[    4.400863] init[74]: code: 90010024 bf61000c 91490a7c 3fa01002 3be00000 7d3e4b78 3bbd0c20 3b600000
[    4.409867] init[74]: code: 3b9d0040 7c7fe02e 2f830000 419e0028 <89230000> 2f890000 41be001c 4b7f6e79

This patch fixes it by preparing complete lines in a buffer and
printing it at once.

Fixes: 88b0fe1757359 ("powerpc: Add show_user_instructions()")
Cc: Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 v3: no change
 v2: Using seq_buf and reworked the loop to avoid redundant prints.

 arch/powerpc/kernel/process.c | 37 +++++++++++++++++++------------------
 1 file changed, 19 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index e108e1ef2b85..2a39f7aca846 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -43,6 +43,7 @@
 #include <linux/uaccess.h>
 #include <linux/elf-randomize.h>
 #include <linux/pkeys.h>
+#include <linux/seq_buf.h>
 
 #include <asm/pgtable.h>
 #include <asm/io.h>
@@ -1303,33 +1304,33 @@ static void show_instructions(struct pt_regs *regs)
 void show_user_instructions(struct pt_regs *regs)
 {
 	unsigned long pc;
-	int i;
+	int n = instructions_to_print;
+	struct seq_buf s;
+	char buf[96]; /* enough for 8 times 9 + 2 chars */
 
 	pc = regs->nip - (instructions_to_print * 3 / 4 * sizeof(int));
 
-	pr_info("%s[%d]: code: ", current->comm, current->pid);
+	seq_buf_init(&s, buf, sizeof(buf));
 
-	for (i = 0; i < instructions_to_print; i++) {
-		int instr;
+	while (n) {
+		int i;
 
-		if (!(i % 8) && (i > 0)) {
-			pr_cont("\n");
-			pr_info("%s[%d]: code: ", current->comm, current->pid);
-		}
+		seq_buf_clear(&s);
 
-		if (probe_kernel_address((const void *)pc, instr)) {
-			pr_cont("XXXXXXXX ");
-		} else {
-			if (regs->nip == pc)
-				pr_cont("<%08x> ", instr);
-			else
-				pr_cont("%08x ", instr);
+		for (i = 0; i < 8 && n; i++, n--, pc += sizeof(int)) {
+			int instr;
+
+			if (probe_kernel_address((const void *)pc, instr)) {
+				seq_buf_puts(&s, "XXXXXXXX ");
+				continue;
+			}
+			seq_buf_printf(&s, regs->nip == pc ? "<%08x> " : "%08x ", instr);
 		}
 
-		pc += sizeof(int);
+		if (!seq_buf_has_overflowed(&s))
+			pr_info("%s[%d]: code: %s\n", current->comm,
+				current->pid, s.buffer);
 	}
-
-	pr_cont("\n");
 }
 
 struct regbit {
-- 
2.13.3