[PATCH 2/2] fs: allow positional I/O on files with unsigned offsets

[Stian Halseth <stian@itx.no>]

pread64(), pwrite64(), preadv() and pwritev() reject a negative position
with -EINVAL before looking up the file, so FOP_UNSIGNED_OFFSET is never
consulted.  lseek() followed by read() on the same descriptor accepts
the position, and rw_verify_area() already handles it, overflow check
included.  Only the positional syscall wrappers reject it up front.

Check the position after the file lookup and let a negative value
through for files with unsigned offsets, as vfs_setpos_cookie() and
rw_verify_area() do.  Behavior for every other file is unchanged.

On sparc64 userspace is mapped above 2^63, so pread() on /proc/PID/mem
always failed with EINVAL.  pldd(1) could not read a target's memory
because of this, and gdb carries a userspace workaround for the same
thing (PR gdb/30525).

Signed-off-by: Stian Halseth <stian@itx.no>
---
 fs/read_write.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 36f3d8e..e46e814 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -753,13 +753,13 @@ SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
 ssize_t ksys_pread64(unsigned int fd, char __user *buf, size_t count,
 		     loff_t pos)
 {
-	if (pos < 0)
-		return -EINVAL;
-
 	CLASS(fd, f)(fd);
 	if (fd_empty(f))
 		return -EBADF;
 
+	if (pos < 0 && !unsigned_offsets(fd_file(f)))
+		return -EINVAL;
+
 	if (fd_file(f)->f_mode & FMODE_PREAD)
 		return vfs_read(fd_file(f), buf, count, &pos);
 
@@ -783,13 +783,13 @@ COMPAT_SYSCALL_DEFINE5(pread64, unsigned int, fd, char __user *, buf,
 ssize_t ksys_pwrite64(unsigned int fd, const char __user *buf,
 		      size_t count, loff_t pos)
 {
-	if (pos < 0)
-		return -EINVAL;
-
 	CLASS(fd, f)(fd);
 	if (fd_empty(f))
 		return -EBADF;
 
+	if (pos < 0 && !unsigned_offsets(fd_file(f)))
+		return -EINVAL;
+
 	if (fd_file(f)->f_mode & FMODE_PWRITE)
 		return vfs_write(fd_file(f), buf, count, &pos);
 
@@ -1123,14 +1123,14 @@ static ssize_t do_preadv(unsigned long fd, const struct iovec __user *vec,
 {
 	ssize_t ret = -EBADF;
 
-	if (pos < 0)
-		return -EINVAL;
-
 	CLASS(fd, f)(fd);
 	if (!fd_empty(f)) {
-		ret = -ESPIPE;
-		if (fd_file(f)->f_mode & FMODE_PREAD)
-			ret = vfs_readv(fd_file(f), vec, vlen, &pos, flags);
+		ret = -EINVAL;
+		if (pos >= 0 || unsigned_offsets(fd_file(f))) {
+			ret = -ESPIPE;
+			if (fd_file(f)->f_mode & FMODE_PREAD)
+				ret = vfs_readv(fd_file(f), vec, vlen, &pos, flags);
+		}
 	}
 
 	if (ret > 0)
@@ -1144,14 +1144,14 @@ static ssize_t do_pwritev(unsigned long fd, const struct iovec __user *vec,
 {
 	ssize_t ret = -EBADF;
 
-	if (pos < 0)
-		return -EINVAL;
-
 	CLASS(fd, f)(fd);
 	if (!fd_empty(f)) {
-		ret = -ESPIPE;
-		if (fd_file(f)->f_mode & FMODE_PWRITE)
-			ret = vfs_writev(fd_file(f), vec, vlen, &pos, flags);
+		ret = -EINVAL;
+		if (pos >= 0 || unsigned_offsets(fd_file(f))) {
+			ret = -ESPIPE;
+			if (fd_file(f)->f_mode & FMODE_PWRITE)
+				ret = vfs_writev(fd_file(f), vec, vlen, &pos, flags);
+		}
 	}
 
 	if (ret > 0)
-- 
2.43.0