[v2,1/2] openat2: drop open_how->__padding field

  The purpose of explicit padding was to allow us to use the space in the
future (C provides no guarantee about the value of padding bytes and
thus userspace could've provided garbage).

However, the downside of explicit padding is that any extension we wish
to add should fit the space exactly (otherwise we may end up with a u16
which will never be used). In addition, the correct error to return for
non-zero padding is not clear (-EINVAL doesn't imply "you're using an
extension field unsupported by this kernel", but -E2BIG seems a bit odd
if the structure size isn't different).

The simplest solution is to just match the design of clone3(2) -- use
u64s for all fields. The extra few-bytes cost of extra fields is not
significant (it's unlikely configuration structs will ever be extremely
large) and it allows for more flag space if necessary. There is also no
need to align the u64s because we will not permit any padding in the
structure.

As openat2(2) is not yet in Linus's tree, we can iron out these minor
warts before we commit to this as a stable ABI.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: David Laight <david.laight@aculab.com>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 fs/open.c                                     |  2 --
 include/uapi/linux/fcntl.h                    | 17 +++++++------
 tools/testing/selftests/openat2/helpers.h     |  7 +++---
 .../testing/selftests/openat2/openat2_test.c  | 24 +++++++------------
 4 files changed, 19 insertions(+), 31 deletions(-)
  

[Cc Arnd for struct layout sanity checking]

On December 20, 2019 3:03:27 PM GMT+01:00, Aleksa Sarai <cyphar@cyphar.com> wrote:
>The purpose of explicit padding was to allow us to use the space in the
>future (C provides no guarantee about the value of padding bytes and
>thus userspace could've provided garbage).
>
>However, the downside of explicit padding is that any extension we wish
>to add should fit the space exactly (otherwise we may end up with a u16
>which will never be used). In addition, the correct error to return for
>non-zero padding is not clear (-EINVAL doesn't imply "you're using an
>extension field unsupported by this kernel", but -E2BIG seems a bit odd
>if the structure size isn't different).
>
>The simplest solution is to just match the design of clone3(2) -- use
>u64s for all fields. The extra few-bytes cost of extra fields is not
>significant (it's unlikely configuration structs will ever be extremely
>large) and it allows for more flag space if necessary. There is also no
>need to align the u64s because we will not permit any padding in the
>structure.
>
>As openat2(2) is not yet in Linus's tree, we can iron out these minor
>warts before we commit to this as a stable ABI.
>
>Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
>Suggested-by: David Laight <david.laight@aculab.com>
>Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
>---
> fs/open.c                                     |  2 --
> include/uapi/linux/fcntl.h                    | 17 +++++++------
> tools/testing/selftests/openat2/helpers.h     |  7 +++---
> .../testing/selftests/openat2/openat2_test.c  | 24 +++++++------------
> 4 files changed, 19 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..5aaadfd79dd5 100644
>--- a/include/uapi/linux/fcntl.h
>+++ b/include/uapi/linux/fcntl.h
>@@ -101,22 +101,21 @@
> #define AT_RECURSIVE		0x8000	/* Apply to the entire subtree */
> 
> /*
>- * Arguments for how openat2(2) should open the target path. If
>@resolve is
>- * zero, then openat2(2) operates very similarly to openat(2).
>+ * Arguments for how openat2(2) should open the target path. If only
>@flags and
>+ * @mode are non-zero, then openat2(2) operates very similarly to
>openat(2).
>  *
>- * However, unlike openat(2), unknown bits in @flags result in -EINVAL
>rather
>- * than being silently ignored. @mode must be zero unless one of
>{O_CREAT,
>- * O_TMPFILE} are set.
>+ * However, unlike openat(2), unknown or invalid bits in @flags result
>in
>+ * -EINVAL rather than being silently ignored. @mode must be zero
>unless one of
>+ *  {O_CREAT, O_TMPFILE} are set.
>  *
>  * @flags: O_* flags.
>  * @mode: O_CREAT/O_TMPFILE file mode.
>  * @resolve: RESOLVE_* flags.
>  */
> struct open_how {
>-	__aligned_u64 flags;
>-	__u16 mode;
>-	__u16 __padding[3]; /* must be zeroed */
>-	__aligned_u64 resolve;
>+	__u64 flags;
>+	__u64 mode;
>+	__u64 resolve;
> };
> 
> #define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
>diff --git a/tools/testing/selftests/openat2/helpers.h
>b/tools/testing/selftests/openat2/helpers.h
>index 43ca5ceab6e3..a6ea27344db2 100644
>--- a/tools/testing/selftests/openat2/helpers.h
>+++ b/tools/testing/selftests/openat2/helpers.h
>@@ -36,10 +36,9 @@
>  * @resolve: RESOLVE_* flags.
>  */
> struct open_how {
>-	__aligned_u64 flags;
>-	__u16 mode;
>-	__u16 __padding[3]; /* must be zeroed */
>-	__aligned_u64 resolve;
>+	__u64 flags;
>+	__u64 mode;
>+	__u64 resolve;
> };
> 
> #define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
>diff --git a/tools/testing/selftests/openat2/openat2_test.c
>b/tools/testing/selftests/openat2/openat2_test.c
>index 0b64fedc008b..b386367c606b 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)
>@@ -57,20 +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. */
>@@ -169,7 +155,7 @@ struct flag_test {
> 	int err;
> };
> 
>-#define NUM_OPENAT2_FLAG_TESTS 21
>+#define NUM_OPENAT2_FLAG_TESTS 23
> 
> void test_openat2_flags(void)
> {
>@@ -214,9 +200,15 @@ void test_openat2_flags(void)
> 		{ .name = "invalid how.mode and O_CREAT",
> 		  .how.flags = O_CREAT,
> 		  .how.mode = 0xFFFF, .err = -EINVAL },
>+		{ .name = "invalid (very large) how.mode and O_CREAT",
>+		  .how.flags = O_CREAT,
>+		  .how.mode = 0xC000000000000000ULL, .err = -EINVAL },
> 		{ .name = "invalid how.mode and O_TMPFILE",
> 		  .how.flags = O_TMPFILE | O_RDWR,
> 		  .how.mode = 0x1337, .err = -EINVAL },
>+		{ .name = "invalid (very large) how.mode and O_TMPFILE",
>+		  .how.flags = O_TMPFILE | O_RDWR,
>+		  .how.mode = 0x0000A00000000000ULL, .err = -EINVAL },
> 
> 		/* ->resolve must only contain RESOLVE_* flags. */
> 		{ .name = "invalid how.resolve and O_RDONLY",