[PATCH v3 2/4] xfs_db: use iocursor type to guess btree geometry if bad magic

[Eric Sandeen <sandeen@sandeen.net>]

From: Darrick J. Wong <darrick.wong@oracle.com>

The function block_to_bt plays an integral role in determining the btree
geometry of a block that we want to manipulate with the debugger.
Normally we use the block magic to find the geometry profile, but if the
magic is bad we'll never find it and return NULL.  The callers of this
function do not check for NULL and crash.

Therefore, if we can't find a geometry profile matching the magic
number, use the iocursor type to guess the profile and scowl about that
to stdout.  This makes it so that even with a corrupt magic we can try
to print the fields instead of crashing the debugger.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v2: be less macro-happy and only evaluate hascrc once
v3: braces around the for loop body
v4: proposed maintainer changes from eric:
    fix comments, add magic assert, keep other asserts which
    were removed for unknown reasons...
---

diff --git a/db/btblock.c b/db/btblock.c
index 835a5f0..b7eacb5 100644
--- a/db/btblock.c
+++ b/db/btblock.c
@@ -25,6 +25,8 @@
 #include "print.h"
 #include "bit.h"
 #include "init.h"
+#include "io.h"
+#include "output.h"
 
 /*
  * Definition of the possible btree block layouts.
@@ -113,22 +115,60 @@ struct xfs_db_btree {
 };
 
 /*
- * Find the right block defintion for a given ondisk block.
- *
- * We use the least significant bit of the magic number as index into
- * the array of block defintions.
+ * Find the right block definition for a given ondisk block.
  */
 static struct xfs_db_btree *
 block_to_bt(
 	struct xfs_btree_block	*bb)
 {
-	struct xfs_db_btree *btp = &btrees[0];
+	struct xfs_db_btree	*btp;
+	uint32_t		magic;
+	bool			crc;
 
-	do {
-		if (be32_to_cpu((bb)->bb_magic) == btp->magic)
+	magic = be32_to_cpu((bb)->bb_magic);
+	for (btp = &btrees[0]; btp->magic != 0; btp++) {
+		if (magic == btp->magic)
+			return btp;
+	}
+
+	/* Magic is invalid/unknown.  Guess based on iocur type */
+	crc = xfs_sb_version_hascrc(&mp->m_sb);
+	switch (iocur_top->typ->typnm) {
+	case TYP_BMAPBTA:
+	case TYP_BMAPBTD:
+		magic = crc ? XFS_BMAP_CRC_MAGIC : XFS_BMAP_MAGIC;
+		break;
+	case TYP_BNOBT:
+		magic = crc ? XFS_ABTB_CRC_MAGIC : XFS_ABTB_MAGIC;
+		break;
+	case TYP_CNTBT:
+		magic = crc ? XFS_ABTC_CRC_MAGIC : XFS_ABTC_MAGIC;
+		break;
+	case TYP_INOBT:
+		magic = crc ? XFS_IBT_CRC_MAGIC : XFS_IBT_MAGIC;
+		break;
+	case TYP_FINOBT:
+		magic = crc ? XFS_FIBT_CRC_MAGIC : XFS_FIBT_MAGIC;
+		break;
+	case TYP_RMAPBT:
+		magic = crc ? XFS_RMAP_CRC_MAGIC : 0;
+		break;
+	case TYP_REFCBT:
+		magic = crc ? XFS_REFC_CRC_MAGIC : 0;
+		break;
+	default:
+		ASSERT(0);
+	}
+
+	ASSERT(magic);
+	dbprintf(_("Bad btree magic 0x%x; coercing to %s.\n"),
+		be32_to_cpu((bb)->bb_magic),
+		iocur_top->typ->name);
+
+	for (btp = &btrees[0]; btp->magic != 0; btp++) {
+		if (magic == btp->magic)
 			return btp;
-		btp++;
-	} while (btp->magic != 0);
+	}
 
 	return NULL;
 }