On 9/7/2018 1:55 PM, Junio C Hamano wrote:
Ben Peart <benpeart@xxxxxxxxxxxxx> writes:
The extension consists of:
- 32-bit offset to the end of the index entries
- 160-bit SHA-1 over the extension types and their sizes (but not
their contents). E.g. if we have "TREE" extension that is N-bytes
long, "REUC" extension that is M-bytes long, followed by "EOIE",
then the hash would be:
SHA-1("TREE" + <binary representation of N> +
"REUC" + <binary representation of M>)
I didn't look at the documentation patch in the larger context, but
please make sure that it is clear to the readers that these fixed
width integers "binary representations" use network byte order.
At the top of the documentation it says "All binary numbers are in
network byte order" and that is not repeated for any of the other
sections that are documenting the file format.
I briefly wondered if the above should include
+ "EOIE" + <binary representation of (32+160)/8 = 24>
as it is pretty much common file format design to include the header
part of the checksum record (with checksum values padded out with NUL
bytes) when you define a record to hold the checksum of the entire
file. Since this does not protect the contents of each section from
bit-flipping, adding the data for EOIE itself in the sum does not
give us much (iow, what I am adding above is a constant that does
not contribute any entropy).
How big is a typical TREE extension in _your_ work repository
housing Windows sources? I am guessing that replacing SHA-1 with
something faster (as this is not about security but is about disk
corruption) and instead hash also the contents of these sections
would NOT help all that much in the performance department, as
having to page them in to read through would already consume
non-trivial amount of time, and that is why you are not hashing the
contents.
The purpose of the SHA isn't to detect disk corruption (we already have
a SHA for the entire index that can serve that purpose) but to help
ensure that this was actually a valid EOIE extension and not a lucky
random set of bytes. I had used leading and trailing signature bytes
along with the length and version bytes to validate it was an actual
EOIE extension but you suggested [1] that I use a SHA of the 4 byte
extension type + 4 byte extension length instead so I rewrote it that way.
[1]
https://public-inbox.org/git/xmqq1sl017dw.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxx/
+ /*
+ * CACHE_EXT_ENDOFINDEXENTRIES must be written as the last entry before the SHA1
s/SHA1/trailing checksum/ or something so that we can withstand
NewHash world order?
I thought about this but in the document elsewhere it refers to it as
"160-bit SHA-1 over the content of the index file before this checksum."
and there are at least a dozen other references to "SHA-1" so I figured
we can fix them all at the same time when we have a new/better name. :-)
+ * so that it can be found and processed before all the index entries are
+ * read.
+ */
+ if (!strip_extensions && offset && !git_env_bool("GIT_TEST_DISABLE_EOIE", 0)) {
+ struct strbuf sb = STRBUF_INIT;
+
+ write_eoie_extension(&sb, &eoie_c, offset);
+ err = write_index_ext_header(&c, NULL, newfd, CACHE_EXT_ENDOFINDEXENTRIES, sb.len) < 0
|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
strbuf_release(&sb);
if (err)
OK.
+#define EOIE_SIZE 24 /* <4-byte offset> + <20-byte hash> */
+#define EOIE_SIZE_WITH_HEADER (4 + 4 + EOIE_SIZE) /* <4-byte signature> + <4-byte length> + EOIE_SIZE */
+
+#ifndef NO_PTHREADS
+static unsigned long read_eoie_extension(void *mmap, size_t mmap_size)
+{
+ /*
+ * The end of index entries (EOIE) extension is guaranteed to be last
+ * so that it can be found by scanning backwards from the EOF.
+ *
+ * "EOIE"
+ * <4-byte length>
+ * <4-byte offset>
+ * <20-byte hash>
+ */
+ const char *index, *eoie = (const char *)mmap + mmap_size - GIT_SHA1_RAWSZ - EOIE_SIZE_WITH_HEADER;
+ uint32_t extsize;
+ unsigned long offset, src_offset;
+ unsigned char hash[GIT_MAX_RAWSZ];
+ git_hash_ctx c;
+
+ /* validate the extension signature */
+ index = eoie;
+ if (CACHE_EXT(index) != CACHE_EXT_ENDOFINDEXENTRIES)
+ return 0;
+ index += sizeof(uint32_t);
+
+ /* validate the extension size */
+ extsize = get_be32(index);
+ if (extsize != EOIE_SIZE)
+ return 0;
+ index += sizeof(uint32_t);
Do we know we have at least 8-byte to consume to perform the above
two checks, or is that something we need to verify at the beginning
of this function? Better yet, as we know that a correct EOIE with
its own header is 28-byte long, we probably should abort if
mmap_size is smaller than that.
I'll add that additional test.
+ /*
+ * Validate the offset we're going to look for the first extension
+ * signature is after the index header and before the eoie extension.
+ */
+ offset = get_be32(index);
+ if ((const char *)mmap + offset < (const char *)mmap + sizeof(struct cache_header))
+ return 0;
Claims that the end is before the beginning, which we reject as bogus. Good.
+ if ((const char *)mmap + offset >= eoie)
+ return 0;
Claims that the end is beyond the EOIE marker we should have placed
after it, which we reject as bogus. Good.
+ index += sizeof(uint32_t);
+
+ /*
+ * The hash is computed over extension types and their sizes (but not
+ * their contents). E.g. if we have "TREE" extension that is N-bytes
+ * long, "REUC" extension that is M-bytes long, followed by "EOIE",
+ * then the hash would be:
+ *
+ * SHA-1("TREE" + <binary representation of N> +
+ * "REUC" + <binary representation of M>)
+ */
+ src_offset = offset;
+ the_hash_algo->init_fn(&c);
+ while (src_offset < mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER) {
+ /* After an array of active_nr index entries,
(Style nit).
+ * there can be arbitrary number of extended
+ * sections, each of which is prefixed with
+ * extension name (4-byte) and section length
+ * in 4-byte network byte order.
+ */
+ uint32_t extsize;
+ memcpy(&extsize, (char *)mmap + src_offset + 4, 4);
+ extsize = ntohl(extsize);
Earlier we were using get_be32() but now we use memcpy with ntohl()?
How are we choosing which one to use?
I literally copy/pasted this logic from the code that actually loads the
extensions then removed the call to load the extension and replaced it
with the call to update the hash. I kept it the same to facilitate
consistency for any future fixes or changes.
I think you meant to cast mmap to (const char *) here. It may make it
easier to write and read if we started this function like so:
static unsigned long read_eoie_extension(void *mmap_, size_t mmap_size)
{
const char *mmap = mmap_;
then we do not have to keep casting mmap and cast to a wrong type by
mistake.
Good suggestion.
+
+ /* verify the extension size isn't so large it will wrap around */
+ if (src_offset + 8 + extsize < src_offset)
+ return 0;
Good.
+ the_hash_algo->update_fn(&c, (const char *)mmap + src_offset, 8);
+
+ src_offset += 8;
+ src_offset += extsize;
+ }
+ the_hash_algo->final_fn(hash, &c);
+ if (hashcmp(hash, (unsigned char *)index))
+ return 0;
+
+ /* Validate that the extension offsets returned us back to the eoie extension. */
+ if (src_offset != mmap_size - the_hash_algo->rawsz - EOIE_SIZE_WITH_HEADER)
+ return 0;
Very good.
+ return offset;
+}
+#endif
Overall it looks like it is carefully done.
Thanks for the careful review!
Thanks.
