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kdump el5 --dump-dmesg 错误
source link: http://abcdxyzk.github.io/blog/2014/08/28/debug-kdump-cannot--dump-dmesg/
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kdump el5 --dump-dmesg 错误
2014-08-28 16:35:00
http://vault.centos.org/5.11/os/SRPMS/kexec-tools-1.102pre-165.el5.src.rpm
这个包的一个patch(kexec-tools-1.102pre-makedumpfile-dump-dmesg.patch)是为了得到dmesg的，
但是它判断dmesg的结束是用logged_chars(看kernel/printk.c)，logged_chars应该是输出的结束，所以不对。
改成log_end就行，
diff --git a/kexec-tools-1.102pre-makedumpfile-dump-dmesg.patch b/kexec-tools-1.102pre-makedumpfile-dump-dmesg.patch
index 3938280..76c402a 100644
--- a/kexec-tools-1.102pre-makedumpfile-dump-dmesg.patch
+++ b/kexec-tools-1.102pre-makedumpfile-dump-dmesg.patch
@@ -68,7 +68,7 @@ diff -up kexec-tools-testing-20070330/makedumpfile/makedumpfile.c.orig kexec-too
+dump_dmesg()
+{
+ int log_buf_len, length_log, length_oldlog, ret = FALSE;
-+ unsigned long log_addr, logged_chars, index;
++ unsigned long log_addr, logged_chars, log_end, index;
+ char *log_buffer = NULL;
+
+ if (!open_files_for_creating_dumpfile())
@@ -101,10 +101,15 @@ diff -up kexec-tools-testing-20070330/makedumpfile/makedumpfile.c.orig kexec-too
+ printf("Failed to get logged_chars.\n");
+ return FALSE;
+ }
++ if (!readmem(VADDR, SYMBOL(log_end), &log_end, sizeof(log_end))) {
++ printf("Failed to get log_end.\n");
++ return FALSE;
++ }
+ DEBUG_MSG("\n");
+ DEBUG_MSG("log_addr : %lx\n", log_addr);
+ DEBUG_MSG("log_buf_len : %d\n", log_buf_len);
+ DEBUG_MSG("logged_chars : %ld\n", logged_chars);
++ DEBUG_MSG("log_end : %ld\n", log_end);
+
+ if ((log_buffer = malloc(log_buf_len)) == NULL) {
+ ERRMSG("Can't allocate memory for log_buf. %s\n",
@@ -112,21 +117,16 @@ diff -up kexec-tools-testing-20070330/makedumpfile/makedumpfile.c.orig kexec-too
+ return FALSE;
+ }
+
-+ if (logged_chars < log_buf_len) {
++ if (log_end < log_buf_len) {
+ index = 0;
-+ length_log = logged_chars;
++ length_log = log_end;
+
+ if(!readmem(VADDR, log_addr, log_buffer, length_log)) {
+ printf("Failed to read dmesg log.\n");
+ goto out;
+ }
+ } else {
-+ if (!readmem(VADDR, SYMBOL(log_end), &index, sizeof(index))) {
-+ printf("Failed to get log_end.\n");
-+ goto out;
-+ }
-+ DEBUG_MSG("log_end : %lx\n", index);
-+ index &= log_buf_len - 1;
++ index = log_end & (log_buf_len - 1);
+ length_log = log_buf_len;
+ length_oldlog = log_buf_len - index;
+如果不修改上面bug，kdump得到vmcore后用 makedumpfile –dump-dmesg 无法解得dmesg，补救办法如下：
kdump得到vmcore后
1、vmlinux没有debuginfo，crash不能运行
2、makedumpfile -F –dump-dmesg vmcore > dmesg 只能显示开头一下部分dmesg （不懂为什么）
解决：
通过/boot/System.map 或者 /proc/kallsyms 找到 log_buf 地址，例如 0xffffffff81a9ac30
gdb vmlinux vmcore
set print repeats 100
set print elements 0
set logging file XXX
set pagination off
set logging on
p {char*} 0xffffffff81a9ac30
quit
---
vi XXX
:%s/\\n/\r/g是另一命令，但不好用
cat /proc/kallsyms | grep log_end
ffffffff81e30de0 b log_end
x/1dw 0xffffffff81e30de0
0xffffffff81e30de0: 85689
x/1xg 0xffffffff81a9ac30
0xffffffff81a9ac30: 0xffffffff81e30ee0
显示最后4000字符
x/5s 0xffffffff81e30ee0+85689-4000Posted by kk
2014-08-28 16:35:00debug, kdump、crash
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