[PATCH v3 2/2] hexdump: add a new dump prefix DUMP_PREFIX_CUSTOM

[thunder . leizhen]

From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>

Currently, function print_hex_dump() supports three dump prefixes:
DUMP_PREFIX_NONE, DUMP_PREFIX_ADDRESS and DUMP_PREFIX_OFFSET. But for some
usage scenarios, they don't work perfectly. For example, dump the content
of one task's stack. In order to quickly identify a stack frame,
DUMP_PREFIX_ADDRESS is preferred. But without boot option no_hash_pointers
, DUMP_PREFIX_ADDRESS just print the 32-bit hash value.

dump memory at sp=ffff800080903aa0:
00000000a00a1d32: 80903ac0 ffff8000 8feeae24 ffffc356
000000007993ef27: 9811c000 ffff0d98 8ad2e500 ffff0d98
00000000b1a0b2de: 80903b30 ffff8000 8ff3a618 ffffc356
... ...
00000000a7a9048b: 9810b3c0 ffff0d98 00000000 00000000
0000000011cda415: 80903cb0 ffff8000 00000000 00000000
000000002dbdf9cd: 981f8400 ffff0d98 00000001 00000000

On the other hand, printing multiple 64-bit addresses is redundant when
the 'sp' value is already printed. Generally, we do not dump more than
64 KiB memory. It is sufficient to print only the lower 16 bits of the
address.

dump memory at sp=ffff800080883a90:
3a90: 80883ac0 ffff8000 3d8e936c ffffbd5b
3aa0: 5833f000 ffff3580 00000001 00000000
3ab0: 40299840 ffff3580 590dfa00 ffff3580
3ac0: 80883b30 ffff8000 3d938b28 ffffbd5b
3ad0: 40877180 ffff3580 590dfa00 ffff3580
3ae0: 4090f600 ffff3580 80883cb0 ffff8000
3af0: 00000010 00000000 00000000 00000000
3b00: 4090f700 ffff3580 00000001 00000000

Let's add DUMP_PREFIX_CUSTOM, allows users to make some adjustments to
their needs.

For example:
pr_info("dump memory at sp=%px:\n", sp);
print_hex_dump(KERN_INFO, "%s%16hx: %s\n",
               DUMP_PREFIX_CUSTOM, 16, 1, sp, 16, false);
print_hex_dump(KERN_INFO, "%s%16x: %s\n",
               DUMP_PREFIX_CUSTOM, 16, 1, sp, 16, false);
print_hex_dump(KERN_INFO, "%s%px: %s\n",
               DUMP_PREFIX_CUSTOM, 16, 1, sp, 16, false);

dump memory at sp=ffff80008091baa0:
            baa0: c0 ba 91 80 00 80 ff ff d4 38 16 ce fc a7 ff ff
        8091baa0: c0 ba 91 80 00 80 ff ff d4 38 16 ce fc a7 ff ff
ffff80008091baa0: c0 ba 91 80 00 80 ff ff d4 38 16 ce fc a7 ff ff

Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
---
 include/linux/printk.h |  3 ++-
 lib/hexdump.c          | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 8ef499ab3c1ed2e..23779dcc4836414 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -704,7 +704,8 @@ extern const struct file_operations kmsg_fops;
 enum {
 	DUMP_PREFIX_NONE,
 	DUMP_PREFIX_ADDRESS,
-	DUMP_PREFIX_OFFSET
+	DUMP_PREFIX_OFFSET,
+	DUMP_PREFIX_CUSTOM
 };
 extern int hex_dump_to_buffer(const void *buf, size_t len, int rowsize,
 			      int groupsize, char *linebuf, size_t linebuflen,
diff --git a/lib/hexdump.c b/lib/hexdump.c
index 1064706d57c15ed..fa4a44543a946b8 100644
--- a/lib/hexdump.c
+++ b/lib/hexdump.c
@@ -232,6 +232,11 @@ EXPORT_SYMBOL(hex_dump_to_buffer);
  * @level: kernel log level (e.g. KERN_DEBUG)
  * @prefix_str: string to prefix each line with;
  *  caller supplies trailing spaces for alignment if desired
+ *  OR
+ *  the custom format string of DUMP_PREFIX_CUSTOM;
+ *  Corresponding to three parameters in fixed order:
+ *  <string: level> <pointer: address> <string: converted data>
+ *  For example: "%s%04hx: %s\n", "%s%.8x: %s\n", "%s%px: %s\n"
  * @prefix_type: controls whether prefix of an offset, address, or none
  *  is printed (%DUMP_PREFIX_OFFSET, %DUMP_PREFIX_ADDRESS, %DUMP_PREFIX_NONE)
  * @rowsize: number of bytes to print per line; must be 16 or 32
@@ -257,6 +262,14 @@ EXPORT_SYMBOL(hex_dump_to_buffer);
  * 0009ab42: 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f  @ABCDEFGHIJKLMNO
  * Example output using %DUMP_PREFIX_ADDRESS and 4-byte mode:
  * ffffffff88089af0: 73727170 77767574 7b7a7978 7f7e7d7c  pqrstuvwxyz{|}~.
+ *
+ * E.g.:
+ *   print_hex_dump(KERN_DEBUG, "%s%04hx: %s\n", DUMP_PREFIX_CUSTOM,
+ *		    16, 1, frame->data, frame->len, false);
+ *   %04hx --> Only the lower 16 bits of the address are printed.
+ *
+ * Example output using %DUMP_PREFIX_CUSTOM and 1-byte mode:
+ * 3aa0: c0 3a 8d 80 00 80 ff ff d4 38 16 1d 94 a6 ff ff
  */
 void print_hex_dump(const char *level, const char *prefix_str, int prefix_type,
 		    int rowsize, int groupsize,
@@ -292,6 +305,9 @@ void print_hex_dump(const char *level, const char *prefix_str, int prefix_type,
 			}
 			printk("%s%s%0*x: %s\n", level, prefix_str, width, i, linebuf);
 			break;
+		case DUMP_PREFIX_CUSTOM:
+			printk(prefix_str, level, ptr + i, linebuf);
+			break;
 		default:
 			printk("%s%s%s\n", level, prefix_str, linebuf);
 			break;
-- 
2.34.1