[v3,2/3] libc: fix sign extension in fallocate()

Submitted by Yuriy Kolerov on Sept. 23, 2015, 12:43 p.m.

Details

Message ID 1443012219-10992-2-git-send-email-yuriy.kolerov@synopsys.com
State New
Headers show

Commit Message

Yuriy Kolerov Sept. 23, 2015, 12:43 p.m.
For common generic syscall ABI fallocate syscall handler in kernel
expects a 64-bit signed arguments for offset and len. However uClibc
has 2 wrappers for this syscall: fallocate and fallocate64.

On 32-bit machines fallocate (not fallocate64) expects 32-bit values of
offset and len. Thus in this case uClibc's fallocate must pass to the
syscall those values with sign extension. High word of 64-bit value must
be 0 or 0xFFFFFFFF depending on sign of the original 32-bit value (offset
or len). It is how sign extansion works - all high bits of the negative
value must be 1.

So on 32-bit machines uClibc's fallocate does sign extension incorrectly
when 32-bit values are passed (offset or len). It just fills the second
word of 64-bit value by zeros. E.g. fallocate works incorrectly when offset
or length is negative value - in this case kernel thinks that positive
values are passed.

Solution is to call fallocate64 from fallocate and pass 32-bit values of
offset and len to fallocate64. off_t type is automatically converted to
off64_t with an appropriate sign extension. Then fallocate64 invokes
kernel's system call properly.

This error is detected in LTP's test kernel/syscalls/fallocate02:

    ----------->8----------
    fallocate(..., 1, -1024, 1024) failed, expected errno:22: TEST_ERRNO=0
    fallocate(..., 1, 1024, -1024) failed, expected errno:22: TEST_ERRNO=0
    fallocate(..., 1, 12288, -1024) failed, expected errno:22: TEST_ERRNO=0
    fallocate(..., 1, -24576, 1024) failed, expected errno:22: TEST_ERRNO=0
    ----------->8----------

fallocate does not emit an error because negative values are passed to the
kernel without sign extension and kernel thinks that it got valid positive
values.

Signed-off-by: Yuriy Kolerov <yuriy.kolerov@synopsys.com>
---
 libc/sysdeps/linux/common/fallocate.c | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

Patch hide | download patch | download mbox

diff --git a/libc/sysdeps/linux/common/fallocate.c b/libc/sysdeps/linux/common/fallocate.c
index b2309e9..0f80fb4 100644
--- a/libc/sysdeps/linux/common/fallocate.c
+++ b/libc/sysdeps/linux/common/fallocate.c
@@ -18,28 +18,20 @@ 
 extern __typeof(fallocate) __libc_fallocate attribute_hidden;
 int attribute_hidden __libc_fallocate(int fd, int mode, __off_t offset, __off_t len)
 {
-	int ret;
-
 # if __WORDSIZE == 32
-	uint32_t off_low = offset;
-	uint32_t len_low = len;
-	/* may assert that these >>31 are 0 */
-	uint32_t zero = 0;
-	INTERNAL_SYSCALL_DECL(err);
-	ret = (int) (INTERNAL_SYSCALL(fallocate, err, 6, fd, mode,
-		__LONG_LONG_PAIR (zero, off_low),
-		__LONG_LONG_PAIR (zero, len_low)));
+	return fallocate64(fd, mode, offset, len);
 # elif __WORDSIZE == 64
+	int ret;
 	INTERNAL_SYSCALL_DECL(err);
 	ret = (int) (INTERNAL_SYSCALL(fallocate, err, 4, fd, mode, offset, len));
-# else
-# error your machine is neither 32 bit or 64 bit ... it must be magical
-# endif
 	if (unlikely(INTERNAL_SYSCALL_ERROR_P (ret, err))) {
 		__set_errno(INTERNAL_SYSCALL_ERRNO (ret, err));
 		ret = -1;
 	}
 	return ret;
+# else
+# error your machine is neither 32 bit or 64 bit ... it must be magical
+# endif
 }
 
 # if defined __UCLIBC_LINUX_SPECIFIC__ && defined __USE_GNU