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Improvements to GPT reading code. 

looking at the gpt code in util.c i found i did not like it at all, a
gpt partition entry is currently 128 bytes, but the spec does not say it
is a fixed value, so the code that reads into a buffer with 512bytes
chunk expecting this to be a multiplier of part_size is imho incorrect.
my fix was to read each partition entry directly into a struct
GPT_part_entry, the advantage is that the code is very simple to read,
the disadvantage it is 128 reads of 128 bytes each, which is
sub-optimal, but i believe readahead will mitigate this a lot.

Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
Luca Berra 2011-06-17 14:41:01 +10:00 committed by NeilBrown
parent 3b7e9d0cbe
commit 73e658d8cc
1 changed files with 9 additions and 18 deletions
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27
util.c
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@ -1119,9 +1119,8 @@ int must_be_container(int fd)
static int get_gpt_last_partition_end(int fd, unsigned long long *endofpart)
{
struct GPT gpt;
unsigned char buf[512];
unsigned char empty_gpt_entry[16]= {0};
struct GPT_part_entry *part;
struct GPT_part_entry part;
unsigned long long curr_part_end;
unsigned all_partitions, entry_size;
unsigned part_nr;
@ -1129,8 +1128,9 @@ static int get_gpt_last_partition_end(int fd, unsigned long long *endofpart)
*endofpart = 0;
BUILD_BUG_ON(sizeof(gpt) != 512);
/* read GPT header */
/* skip protective MBR */
lseek(fd, 512, SEEK_SET);
/* read GPT header */
if (read(fd, &gpt, 512) != 512)
return 0;
@ -1147,28 +1147,19 @@ static int get_gpt_last_partition_end(int fd, unsigned long long *endofpart)
entry_size > 512)
return -1;
/* read first GPT partition entries */
if (read(fd, buf, 512) != 512)
return 0;
part = (struct GPT_part_entry*)buf;
for (part_nr=0; part_nr < all_partitions; part_nr++) {
/* read partition entry */
if (read(fd, &part, entry_size) != (ssize_t)entry_size)
return 0;
/* is this valid partition? */
if (memcmp(part->type_guid, empty_gpt_entry, 16) != 0) {
if (memcmp(part.type_guid, empty_gpt_entry, 16) != 0) {
/* check the last lba for the current partition */
curr_part_end = __le64_to_cpu(part->ending_lba);
curr_part_end = __le64_to_cpu(part.ending_lba);
if (curr_part_end > *endofpart)
*endofpart = curr_part_end;
}
part = (struct GPT_part_entry*)((unsigned char*)part + entry_size);
if ((unsigned char *)part >= buf + 512) {
if (read(fd, buf, 512) != 512)
return 0;
part = (struct GPT_part_entry*)buf;
}
}
return 1;
}