[v2] sysdeps/posix/posix_fallocate*: Make emulated posix_fallocate() work properly
Commit Message
On 14/01/2020 22:31, Xiao Yang wrote:
> Emulated posix_fallocate() only writes data in one block if block
> size is 4096, offset is 4095 and len is 2. The emulated code should
> write data in two blocks in the case because it actually crosses two
> blocks.
Thanks for catching it, comments below.
>
> Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
We do not use signed-off, but Copyright assignments. I am not sure
this particular fix can be considered trivial.
> ---
> sysdeps/posix/posix_fallocate.c | 17 +++++++++++++++++
> sysdeps/posix/posix_fallocate64.c | 17 +++++++++++++++++
> 2 files changed, 34 insertions(+)
>
> diff --git a/sysdeps/posix/posix_fallocate.c b/sysdeps/posix/posix_fallocate.c
> index e7fccfc1c8..22e5fea091 100644
> --- a/sysdeps/posix/posix_fallocate.c
> +++ b/sysdeps/posix/posix_fallocate.c
> @@ -93,6 +93,23 @@ posix_fallocate (int fd, __off_t offset, __off_t len)
> increment = 4096;
> }
>
> + if (offset % increment + len % increment > increment)
> + {
> + if (offset < st.st_size)
> + {
> + unsigned char b;
> + ssize_t rdsize = __pread (fd, &b, 1, offset);
> + if (rdsize < 0)
> + return errno;
> + if (rdsize == 1 && b != 0)
> + goto next;
> + }
> +
> + if (__pwrite (fd, "", 1, offset) != 1)
> + return errno;
> + }
> +
> +next:
The patch looks ok, although I would prefer if we factor the logic where
or not to write the byte on its own function. Something like:
> /* Write a null byte to every block. This is racy; we currently
> lack a better option. Compare-and-swap against a file mapping
> might additional local races, but requires interposition of a
> diff --git a/sysdeps/posix/posix_fallocate64.c b/sysdeps/posix/posix_fallocate64.c
> index f9d4fe5ca3..1c46b186b6 100644
> --- a/sysdeps/posix/posix_fallocate64.c
> +++ b/sysdeps/posix/posix_fallocate64.c
> @@ -93,6 +93,23 @@ __posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len)
> increment = 4096;
> }
>
> + if (offset % increment + len % increment > increment)
> + {
> + if (offset < st.st_size)
> + {
> + unsigned char b;
> + ssize_t rdsize = __libc_pread64 (fd, &b, 1, offset);
> + if (rdsize < 0)
> + return errno;
> + if (rdsize == 1 && b != 0)
> + goto next;
> + }
> +
> + if (__libc_pwrite64 (fd, "", 1, offset) != 1)
> + return errno;
> + }
> +
> +next:
> /* Write a null byte to every block. This is racy; we currently
> lack a better option. Compare-and-swap against a file mapping
> might address local races, but requires interposition of a signal
>
@@ -23,6 +23,26 @@
#include <sys/stat.h>
#include <sys/statfs.h>
+static inline int
+write_byte (int fd, off_t offset, off_t st_size)
+{
+ if (offset < st_size)
+ {
+ unsigned char b;
+ ssize_t rdsize = __libc_pread64 (fd, &b, 1, offset);
+ if (rdsize < 0)
+ return errno;
+ /* If there is a non-zero byte, the block must have been
+ allocated already. */
+ if (rdsize == 1 && b != 0)
+ return 0;
+ }
+
+ if (__libc_pwrite64 (fd, "", 1, offset) != 1)
+ return errno;
+ return 0;
+}
+
/* Reserve storage for the data of the file associated with FD. This
emulation is far from perfect, but the kernel cannot do not much
better for network file systems, either. */
@@ -31,6 +51,7 @@ int
posix_fallocate (int fd, __off_t offset, __off_t len)
{
struct stat64 st;
+ int r;
if (offset < 0 || len < 0)
return EINVAL;
@@ -97,25 +118,21 @@ posix_fallocate (int fd, __off_t offset, __off_t len)
lack a better option. Compare-and-swap against a file mapping
might additional local races, but requires interposition of a
signal handler to catch SIGBUS. */
+
+ if (offset % increment + len % increment > increment)
+ {
+ r = write_byte (fd, offset, st.st_size);
+ if (r != 0)
+ return r;
+ }
+
for (offset += (len - 1) % increment; len > 0; offset += increment)
{
len -= increment;
- if (offset < st.st_size)
- {
- unsigned char c;
- ssize_t rsize = __pread (fd, &c, 1, offset);
-
- if (rsize < 0)
- return errno;
- /* If there is a non-zero byte, the block must have been
- allocated already. */
- else if (rsize == 1 && c != 0)
- continue;
- }
-
- if (__pwrite (fd, "", 1, offset) != 1)
- return errno;
+ r = write_byte (fd, offset, st.st_size);
+ if (r != 0)
+ return r;
}
return 0;