[PATCH v2] pack-objects: use size_t for streamed large blobs

[Elia Pinto <gitter.spiros@gmail.com>]

In builtin/pack-objects.c, write_no_reuse_object() handles blobs
exceeding core.bigFileThreshold by writing them in a streaming path. It
takes the size from st->size (size_t) and assigns it to an "unsigned
long" variable, which is then written to the pack object header, and
write_large_blob_data() adds up the compressed length in an "unsigned
long" as well.

On platforms where "long" is 32 bits, handling a blob of 4GiB or more
truncates the size in the pack header while the data decompresses to the
full size, corrupting the object. If the compressed data also exceeds
4GiB, the length accumulation overflows "unsigned long", misaligning the
offsets of all subsequent objects in the index.

When running repack or gc on a loose large blob, the original loose copy
is pruned, leaving only the corrupted packed copy and resulting in data
loss. Truncating st->size to 16 bits, with a 100KiB loose blob reachable
from a commit and "git -c core.bigFileThreshold=1k repack -a -d", leaves
a corrupt packed object and no loose copy.

Switch the streamed path to use size_t for the object size, and off_t
for the length of the compressed data, which write_large_blob_data()
adds up while it writes the stream piece by piece, and for
write_no_reuse_object()'s return value, like write_reuse_object()
already does. Other truncations in pack-objects are not reachable
because core.bigFileThreshold is itself an "unsigned long".

Triggering this requires processing an object of 4GiB or more on a
platform where "long" is 32 bits, so no test is added.

Helped-by: Simon Richter <Simon.Richter@hogyros.de>
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
---
Changes since v1: use off_t for the length of the streamed data,
following Simon's comment.

 builtin/pack-objects.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index ca3a891dfb..24203b5359 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -412,13 +412,13 @@ static unsigned long do_compress(void **pptr, unsigned long size)
 	return stream.total_out;
 }
 
-static unsigned long write_large_blob_data(struct odb_stream *st, struct hashfile *f,
-					   const struct object_id *oid)
+static off_t write_large_blob_data(struct odb_stream *st, struct hashfile *f,
+				   const struct object_id *oid)
 {
 	git_zstream stream;
 	unsigned char ibuf[1024 * 16];
 	unsigned char obuf[1024 * 16];
-	unsigned long olen = 0;
+	off_t olen = 0;
 	struct repo_config_values *cfg = repo_config_values(the_repository);
 
 	git_deflate_init(&stream, cfg->pack_compression_level);
@@ -513,10 +513,11 @@ static inline int oe_size_greater_than(struct packing_data *pack,
 }
 
 /* Return 0 if we will bust the pack-size limit */
-static unsigned long write_no_reuse_object(struct hashfile *f, struct object_entry *entry,
-					   unsigned long limit, int usable_delta)
+static off_t write_no_reuse_object(struct hashfile *f, struct object_entry *entry,
+				   unsigned long limit, int usable_delta)
 {
-	unsigned long size, datalen;
+	size_t size;
+	off_t datalen;
 	unsigned char header[MAX_PACK_OBJECT_HEADER],
 		      dheader[MAX_PACK_OBJECT_HEADER];
 	unsigned hdrlen;

Range-diff against v1:
1:  45e45c5b75 ! 1:  185b9da891 pack-objects: use size_t for streamed large blobs
    @@ Commit message
         from a commit and "git -c core.bigFileThreshold=1k repack -a -d", leaves
         a corrupt packed object and no loose copy.
     
    -    Switch the streamed path to use size_t for size and datalen, make
    -    write_large_blob_data() add up and return the length as a size_t, and
    -    use off_t for write_no_reuse_object()'s return value. Other truncations
    -    in pack-objects are not reachable because core.bigFileThreshold is
    -    itself an "unsigned long".
    +    Switch the streamed path to use size_t for the object size, and off_t
    +    for the length of the compressed data, which write_large_blob_data()
    +    adds up while it writes the stream piece by piece, and for
    +    write_no_reuse_object()'s return value, like write_reuse_object()
    +    already does. Other truncations in pack-objects are not reachable
    +    because core.bigFileThreshold is itself an "unsigned long".
     
         Triggering this requires processing an object of 4GiB or more on a
         platform where "long" is 32 bits, so no test is added.
     
    +    Helped-by: Simon Richter <Simon.Richter@hogyros.de>
         Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
     
      ## builtin/pack-objects.c ##
    @@ builtin/pack-objects.c: static unsigned long do_compress(void **pptr, unsigned l
      
     -static unsigned long write_large_blob_data(struct odb_stream *st, struct hashfile *f,
     -					   const struct object_id *oid)
    -+static size_t write_large_blob_data(struct odb_stream *st, struct hashfile *f,
    -+				    const struct object_id *oid)
    ++static off_t write_large_blob_data(struct odb_stream *st, struct hashfile *f,
    ++				   const struct object_id *oid)
      {
      	git_zstream stream;
      	unsigned char ibuf[1024 * 16];
      	unsigned char obuf[1024 * 16];
     -	unsigned long olen = 0;
    -+	size_t olen = 0;
    ++	off_t olen = 0;
      	struct repo_config_values *cfg = repo_config_values(the_repository);
      
      	git_deflate_init(&stream, cfg->pack_compression_level);
    @@ builtin/pack-objects.c: static inline int oe_size_greater_than(struct packing_da
     +				   unsigned long limit, int usable_delta)
      {
     -	unsigned long size, datalen;
    -+	size_t size, datalen;
    ++	size_t size;
    ++	off_t datalen;
      	unsigned char header[MAX_PACK_OBJECT_HEADER],
      		      dheader[MAX_PACK_OBJECT_HEADER];
      	unsigned hdrlen;

base-commit: 50fe5cc63f9cd6e89b87a2c9ccf96a444f4cfdb7
-- 
2.53.0