From: short <>
Date: Tue, 6 Aug 2002 14:30:49 +0000 (+0000)
Subject: +README/"Linux kernel flaw" and its handling (last sector failed reads)
X-Git-Url: http://git.jankratochvil.net/?p=badblock-guess.git;a=commitdiff_plain;h=6b440e700542984772b723f222f2d6d3426210b9;hp=bf17104b55bd5232d6564047ecd6dc8cbb397328

+README/"Linux kernel flaw" and its handling (last sector failed reads)
---

diff --git a/README b/README
index fee9795..b02fc61 100644
--- a/README
+++ b/README
@@ -91,6 +91,21 @@ performance about 90KB/s (approx. 7.5GB/24hours if no badblocks read retrying
 is needed). YMMV.
 
 
+Linux kernel flaw
+-----------------
+
+Linux kernels have internal blocksize 1KB but the partitions can be sized by
+512 sectors and thus the last odd partition sector gets inaccessible.
+This isn't a problem for a the disk devices as AFAIK all the disks have always
+even number of sectors.
+
+This program is aware of this flaw and it will not report last such sector as
+BAD - just the appropriate warning is supplied (to stderr). This may be visible
+for example while recovering NTFS partition with odd cylinder number (=>odd
+sector count) as NTFS uses the last partition sector for its superblock backup.
+Fortunately CHKDSK will fix it back, of course.
+
+
 Compilation
 -----------
 
diff --git a/badblock-guess.c b/badblock-guess.c
index f715cd1..e7dacd2 100644
--- a/badblock-guess.c
+++ b/badblock-guess.c
@@ -274,6 +274,13 @@ ext2_loff_t gotext2_loff;
 		exit(EXIT_FAILURE);
 		}
 	if (len!=(gotssize=write(dst_fd,buf,len))) {
+		/* see README/"Linux kernel flaw" */
+		/* no "start==src_len-1" checking here, we may write larger blocks! */
+		if (gotssize==((src_len&~1)-start)*BLOCK
+				/* -1 instead of 0 is given when no bytes were written */
+				|| (gotssize==-1 && start==src_len-1 && end==src_len))
+			return;	/* suppress error message */
+
 		fprintf(stderr,"write(\"%s\",@%llu-@%llu=%llu=%lluB)=%lld: %m\n",dst_name,
 				(unsigned long long)start,
 				(unsigned long long)end,
@@ -284,11 +291,29 @@ ext2_loff_t gotext2_loff;
 		}
 }
 
+static void range_zero(ext2_loff_t start,ext2_loff_t end)
+{
+static const unsigned char buf_zero[BUF_ZERO_BLOCKS*BLOCK];
+
+	g_return_if_fail(start<end);
+
+	while (start<end) {
+		ext2_loff_t mid=((end-start)>BUF_ZERO_BLOCKS ? start+BUF_ZERO_BLOCKS : end);
+		g_assert(mid>=start);
+		g_assert(mid<=end);
+
+		write_dst(start,mid,buf_zero);
+
+		start=mid;
+		}
+}
+
 static void process(ext2_loff_t start,ext2_loff_t end)
 {
 unsigned char block_buf[BLOCK];
 ext2_loff_t bads;
 ext2_loff_t gotext2_loff;
+ssize_t gotssize;
 
 	g_return_if_fail(start<end);
 	g_return_if_fail(end<=src_len);
@@ -308,9 +333,18 @@ restart:	/* continues when the block was successfuly read */
 		exit(EXIT_FAILURE);
 		}
 	stat_todo--;	/* for the forthcoming read() */
-	if (!simulate_bads && sizeof(block_buf)==read(src_fd,block_buf,sizeof(block_buf))) {
+	if (!simulate_bads && (sizeof(block_buf)==(gotssize=read(src_fd,block_buf,sizeof(block_buf)))
+					/* see README/"Linux kernel flaw" */
+					|| (start==src_len-1 && gotssize==((src_len&~1)-start)*BLOCK))
+			) {
 		/* success read */
-		write_dst(start,start+1,block_buf);
+		if (gotssize==sizeof(block_buf))
+			write_dst(start,start+1,block_buf);
+		else {
+			/* handle README/"Linux kernel flaw" */
+			fprintf(stderr,"WARNING: Ignored disk-last sector read failure; see README/\"Linux kernel flaw\"\n");
+			range_zero(start,start+1);
+			}
 		start++;
 		if (start>=end)	/* finished */
 			return;
@@ -337,23 +371,6 @@ ext2_loff_t mid=(start+end)/2;
 		}
 }
 
-static void range_zero(ext2_loff_t start,ext2_loff_t end)
-{
-static const unsigned char buf_zero[BUF_ZERO_BLOCKS*BLOCK];
-
-	g_return_if_fail(start<end);
-
-	while (start<end) {
-		ext2_loff_t mid=((end-start)>BUF_ZERO_BLOCKS ? start+BUF_ZERO_BLOCKS : end);
-		g_assert(mid>=start);
-		g_assert(mid<=end);
-
-		write_dst(start,mid,buf_zero);
-
-		start=mid;
-		}
-}
-
 static void finish(void)
 {
 struct range *todo_linear=tree_linearize(todo_tree),*todol;