Re: [PATCH 2/2] openat2: drop open_how->__padding field

[Christian Brauner]

On Thu, Dec 19, 2019 at 09:55:30PM +1100, Aleksa Sarai 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.
> 
> 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.
> 
> Suggested-by: David Laight <david.laight@xxxxxxxxxx>
> Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>

Fine with me.
Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>

> ---
>  fs/open.c                                     |  2 --
>  include/uapi/linux/openat2.h                  |  3 +--
>  tools/testing/selftests/openat2/helpers.h     |  3 +--
>  .../testing/selftests/openat2/openat2_test.c  | 24 +++++++------------
>  4 files changed, 10 insertions(+), 22 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/openat2.h b/include/uapi/linux/openat2.h
> index 19ef775e8e5e..76fad4ada2d4 100644
> --- a/include/uapi/linux/openat2.h
> +++ b/include/uapi/linux/openat2.h
> @@ -16,8 +16,7 @@
>   */
>  struct open_how {
>  	__aligned_u64 flags;
> -	__u16 mode;
> -	__u16 __padding[3]; /* must be zeroed */
> +	__aligned_u64 mode;
>  	__aligned_u64 resolve;
>  };
>  
> diff --git a/tools/testing/selftests/openat2/helpers.h b/tools/testing/selftests/openat2/helpers.h
> index 43ca5ceab6e3..d756775d0725 100644
> --- a/tools/testing/selftests/openat2/helpers.h
> +++ b/tools/testing/selftests/openat2/helpers.h
> @@ -37,8 +37,7 @@
>   */
>  struct open_how {
>  	__aligned_u64 flags;
> -	__u16 mode;
> -	__u16 __padding[3]; /* must be zeroed */
> +	__aligned_u64 mode;
>  	__aligned_u64 resolve;
>  };
>  
> 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",
> -- 
> 2.24.0
>