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Subject[PATCH] openat2: switch to __attribute__((packed)) for open_how
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The design of the original open_how struct layout was such that it
ensured that there would be no un-labelled (and thus potentially
non-zero) padding to avoid issues with struct expansion, as well as
providing a uniform representation on all architectures (to avoid
complications with OPEN_HOW_SIZE versioning).

However, there were a few other desirable features which were not
fulfilled by the previous struct layout:

* Adding new features (other than new flags) should always result in
the struct getting larger. However, by including a padding field, it
was possible for new fields to be added without expanding the
structure. This would somewhat complicate version-number based
checking of feature support.

* A non-zero bit in __padding yielded -EINVAL when it should arguably
have been -E2BIG (because the padding bits are effectively
yet-to-be-used fields). However, the semantics are not entirely clear
because userspace may expect -E2BIG to only signify that the
structure is too big. It's much simpler to just provide the guarantee
that new fields will always result in a struct size increase, and
-E2BIG indicates you're using a field that's too recent for an older
kernel.

* While the alignment for u64s was manually backed by extra padding
fields, some languages (such as Rust) do not currently support
enforcing alignment of struct field members.

* The padding wasted space needlessly, and would very likely not be
used up entirely by future extensions for a long time (because it
couldn't fit a u64).

While none of these outstanding issues are deal-breakers, we can iron
out these warts before openat2(2) lands in Linus's tree. Instead of
using alignment and padding, we simply pack the structure with
__attribute__((packed)). Rust supports #[repr(packed)] and it removes
all of the issues with having explicit padding.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
fs/open.c | 2 --
include/uapi/linux/fcntl.h | 11 +++++------
tools/testing/selftests/openat2/helpers.h | 11 +++++------
tools/testing/selftests/openat2/openat2_test.c | 18 +-----------------
4 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 50a46501bcc9..8cdb2b675867 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -993,8 +993,6 @@ static inline int build_open_flags(const struct open_how *how,
return -EINVAL;
if (how->resolve & ~VALID_RESOLVE_FLAGS)
return -EINVAL;
- if (memchr_inv(how->__padding, 0, sizeof(how->__padding)))
- return -EINVAL;

/* Deal with the mode. */
if (WILL_CREATE(flags)) {
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index d886bdb585e4..0e070c7f568a 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -109,17 +109,16 @@
* O_TMPFILE} are set.
*
* @flags: O_* flags.
- * @mode: O_CREAT/O_TMPFILE file mode.
* @resolve: RESOLVE_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
*/
struct open_how {
- __aligned_u64 flags;
+ __u64 flags;
+ __u64 resolve;
__u16 mode;
- __u16 __padding[3]; /* must be zeroed */
- __aligned_u64 resolve;
-};
+} __attribute__((packed));

-#define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */
+#define OPEN_HOW_SIZE_VER0 18 /* sizeof first published struct */
#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0

/* how->resolve flags for openat2(2). */
diff --git a/tools/testing/selftests/openat2/helpers.h b/tools/testing/selftests/openat2/helpers.h
index 43ca5ceab6e3..eb1535c8fa2e 100644
--- a/tools/testing/selftests/openat2/helpers.h
+++ b/tools/testing/selftests/openat2/helpers.h
@@ -32,17 +32,16 @@
* O_TMPFILE} are set.
*
* @flags: O_* flags.
- * @mode: O_CREAT/O_TMPFILE file mode.
* @resolve: RESOLVE_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
*/
struct open_how {
- __aligned_u64 flags;
+ __u64 flags;
+ __u64 resolve;
__u16 mode;
- __u16 __padding[3]; /* must be zeroed */
- __aligned_u64 resolve;
-};
+} __attribute__((packed));

-#define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */
+#define OPEN_HOW_SIZE_VER0 18 /* sizeof first published struct */
#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0

bool needs_openat2(const struct open_how *how);
diff --git a/tools/testing/selftests/openat2/openat2_test.c b/tools/testing/selftests/openat2/openat2_test.c
index 0b64fedc008b..cbf95d160b1b 100644
--- a/tools/testing/selftests/openat2/openat2_test.c
+++ b/tools/testing/selftests/openat2/openat2_test.c
@@ -40,7 +40,7 @@ struct struct_test {
int err;
};

-#define NUM_OPENAT2_STRUCT_TESTS 10
+#define NUM_OPENAT2_STRUCT_TESTS 7
#define NUM_OPENAT2_STRUCT_VARIATIONS 13

void test_openat2_struct(void)
@@ -57,22 +57,6 @@ void test_openat2_struct(void)
.arg.inner.flags = O_RDONLY,
.size = sizeof(struct open_how_ext) },

- /* Normal struct with broken padding. */
- { .name = "normal struct (non-zero padding[0])",
- .arg.inner.flags = O_RDONLY,
- .arg.inner.__padding = {0xa0, 0x00, 0x00},
- .size = sizeof(struct open_how_ext), .err = -EINVAL },
- { .name = "normal struct (non-zero padding[1])",
- .arg.inner.flags = O_RDONLY,
- .arg.inner.__padding = {0x00, 0x1a, 0x00},
- .size = sizeof(struct open_how_ext), .err = -EINVAL },
- { .name = "normal struct (non-zero padding[2])",
- .arg.inner.flags = O_RDONLY,
- .arg.inner.__padding = {0x00, 0x00, 0xef},
- .size = sizeof(struct open_how_ext), .err = -EINVAL },
-
- /* TODO: Once expanded, check zero-padding. */
-
/* Smaller than version-0 struct. */
{ .name = "zero-sized 'struct'",
.arg.inner.flags = O_RDONLY, .size = 0, .err = -EINVAL },
base-commit: 912dfe068c43fa13c587b8d30e73d335c5ba7d44
--
2.24.0

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