Using sendfile with below small program to get MD5 sums of some files,
it appear that big files (over 64kbytes with 4k pages system) get a
wrong MD5 sum while small files get the correct sum.
This program uses sendfile() to send a file to an AF_ALG socket
for hashing.
/* md5sum2.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <linux/if_alg.h>
int main(int argc, char **argv)
{
int sk = socket(AF_ALG, SOCK_SEQPACKET, 0);
struct stat st;
struct sockaddr_alg sa = {
.salg_family = AF_ALG,
.salg_type = "hash",
.salg_name = "md5",
};
int n;
bind(sk, (struct sockaddr*)&sa, sizeof(sa));
for (n = 1; n < argc; n++) {
int size;
int offset = 0;
char buf[4096];
int fd;
int sko;
int i;
fd = open(argv[n], O_RDONLY);
sko = accept(sk, NULL, 0);
fstat(fd, &st);
size = st.st_size;
sendfile(sko, fd, &offset, size);
size = read(sko, buf, sizeof(buf));
for (i = 0; i < size; i++)
printf("%2.2x", buf[i]);
printf(" %s\n", argv[n]);
close(fd);
close(sko);
}
exit(0);
}
Test below is done using official linux patch files. First result is
with a software based md5sum. Second result is with the program above.
root@vgoip:~# ls -l patch-3.6.*
-rw-r--r-- 1 root root 64011 Aug 24 12:01 patch-3.6.2.gz
-rw-r--r-- 1 root root 94131 Aug 24 12:01 patch-3.6.3.gz
root@vgoip:~# md5sum patch-3.6.*
b3ffb9848196846f31b2ff133d2d6443 patch-3.6.2.gz
c5e8f687878457db77cb7158c38a7e43 patch-3.6.3.gz
root@vgoip:~# ./md5sum2 patch-3.6.*
b3ffb9848196846f31b2ff133d2d6443 patch-3.6.2.gz
5fd77b24e68bb24dcc72d6e57c64790e patch-3.6.3.gz
After investivation, it appears that sendfile() sends the files by blocks
of 64kbytes (16 times PAGE_SIZE). The problem is that at the end of each
block, the SPLICE_F_MORE flag is missing, therefore the hashing operation
is reset as if it was the end of the file.
This patch adds SPLICE_F_MORE to the flags when more data is pending.
With the patch applied, we get the correct sums:
root@vgoip:~# md5sum patch-3.6.*
b3ffb9848196846f31b2ff133d2d6443 patch-3.6.2.gz
c5e8f687878457db77cb7158c38a7e43 patch-3.6.3.gz
root@vgoip:~# ./md5sum2 patch-3.6.*
b3ffb9848196846f31b2ff133d2d6443 patch-3.6.2.gz
c5e8f687878457db77cb7158c38a7e43 patch-3.6.3.gz
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
---
v2: no change, only new commit text
v3: comment added iaw Jens Axboe suggestion
fs/splice.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/fs/splice.c b/fs/splice.c
index 476024b..bfe62ae 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1161,7 +1161,7 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
long ret, bytes;
umode_t i_mode;
size_t len;
- int i, flags;
+ int i, flags, more;
/*
* We require the input being a regular file, as we don't want to
@@ -1204,6 +1204,7 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
* Don't block on output, we have to drain the direct pipe.
*/
sd->flags &= ~SPLICE_F_NONBLOCK;
+ more = sd->flags & SPLICE_F_MORE;
while (len) {
size_t read_len;
@@ -1217,6 +1218,15 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
sd->total_len = read_len;
/*
+ * If more data is pending, set SPLICE_F_MORE
+ * If this is the last data and SPLICE_F_MORE was not set
+ * initially, clears it.
+ */
+ if (read_len < len)
+ sd->flags |= SPLICE_F_MORE;
+ else if (!more)
+ sd->flags &= ~SPLICE_F_MORE;
+ /*
* NOTE: nonblocking mode only applies to the input. We
* must not do the output in nonblocking mode as then we
* could get stuck data in the internal pipe:
--
2.1.0
--
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