copy_file_range (which calls generic_copy_file_checks) uses the
inode file size to adjust the copy count parameter. This breaks
with special filesystems like procfs/sysfs, where the file size
appears to be zero, but content is actually returned when a read
operation is performed.
This commit ignores the source file size, and makes copy_file_range
match the end of file behaviour documented in POSIX's "read",
where 0 is returned to mark EOF. This would allow "cp" and other
standard tools to make use of copy_file_range with the exact same
behaviour as they had in the past.
Fixes: 96e6e8f4a68d ("vfs: add missing checks to copy_file_range")
Signed-off-by: Nicolas Boichat <drink...@chromium.org>
---
This can be reproduced with this simple test case:
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
int
main(int argc, char **argv)
{
int fd_in, fd_out;
loff_t ret;
fd_in = open("/proc/version", O_RDONLY);
fd_out = open("version", O_CREAT | O_WRONLY | O_TRUNC, 0644);
do {
ret = copy_file_range(fd_in, NULL, fd_out, NULL, 1024, 0);
printf("%d bytes copied\n", (int)ret);
} while (ret > 0);
return 0;
}
Without this patch, `version` output file is empty, and no bytes
are copied:
0 bytes copied
With this patch, the loop runs twice and the content of the file
is copied:
315 bytes copied
0 bytes copied
We hit this issue when upgrading Go compiler from 1.13 to 1.15 [1],
as we use Go's `io.Copy` to copy the content of
`/sys/kernel/debug/tracing/trace` to a temporary file.
Under the hood, Go 1.15 uses `copy_file_range` syscall to optimize the
copy operation. However, that fails to copy any content when the input
file is from sysfs/tracefs, with an apparent size of 0 (but there is
still content when you `cat` it, of course).
[1] http://issuetracker.google.com/issues/178332739
fs/read_write.c | 8 --------
1 file changed, 8 deletions(-)
diff --git a/fs/read_write.c b/fs/read_write.c
index 75f764b43418..7236146f6ad7 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1424,7 +1424,6 @@ static int generic_copy_file_checks(struct file *file_in,
loff_t pos_in,
struct inode *inode_in = file_inode(file_in);
struct inode *inode_out = file_inode(file_out);
uint64_t count = *req_count;
- loff_t size_in;
int ret;
ret = generic_file_rw_checks(file_in, file_out);
@@ -1442,13 +1441,6 @@ static int generic_copy_file_checks(struct file
*file_in, loff_t pos_in,
if (pos_in + count < pos_in || pos_out + count < pos_out)
return -EOVERFLOW;
- /* Shorten the copy to EOF */
- size_in = i_size_read(inode_in);
- if (pos_in >= size_in)
- count = 0;
- else
- count = min(count, size_in - (uint64_t)pos_in);
-
ret = generic_write_check_limits(file_out, pos_out, &count);
if (ret)
return ret;
--
2.30.0.280.ga3ce27912f-goog
