[Crash-utility] [PATCH 3/5] diskname, zram: fix fault error when reading zram disk with no symbol information

[HATAYAMA Daisuke]

When no zram symbol information is initialized, reading memory swapped
out into zram disk results in fault error as follows:

    crash> rd -u 0x00007f1cf6c37000

    rd: invalid structure member offset: zram_compressor
        FILE: diskdump.c  LINE: 2753  FUNCTION: try_zram_decompress()

    [./crash] error trace: 47a7b1 => 5766eb => 5401b7 => 540146

      540146: OFFSET_verify.part.0+70
      5401b7: OFFSET_verify+39
      5766eb: try_zram_decompress+635
      47a7b1: readmem+273

    rd: invalid structure member offset: zram_compressor
        FILE: diskdump.c  LINE: 2753  FUNCTION: try_zram_decompress()

Before zram support, trying to read memory that is swapped out
resulted in inaccessible error as follows:

    crash> rd -u 0x00007f1cf6c37000
    rd: invalid user virtual address: 7f1cf6c37000  type: "64-bit UVADDR"

This behavior is problematic for crash gcore command to support zram.
The fault error terminates gcore command and then generating core file
fails; this is regression. On the other hand, in the previous one,
gcore command can continue by writing zero pages.

Signed-off-by: HATAYAMA Daisuke <d.hatayama at fujitsu.com>
---
 diskdump.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/diskdump.c b/diskdump.c
index 047d8ad..304e9a1 100644
--- a/diskdump.c
+++ b/diskdump.c
@@ -2749,8 +2749,11 @@ try_zram_decompress(ulonglong pte_val, unsigned char *buf, ulong len, ulonglong
 		return 0;
 	}
 
-	if (INVALID_MEMBER(zram_compressor))
+	if (INVALID_MEMBER(zram_compressor)) {
 		zram_init();
+		if (INVALID_MEMBER(zram_compressor))
+			return 0;
+	}
 
 	if (CRASHDEBUG(2))
 		error(WARNING, "this page has swapped to zram\n");
-- 
1.8.3.1