Re: [musl] [PATCH 1/1] resubmitting old statx patch with changes

[Rich Felker <dalias@libc.org>]

On Sat, Aug 27, 2022 at 03:57:52PM +0100, Duncan Bellamy wrote:
> ---
>  include/sys/stat.h | 49 ++++++++++++++++++++++++++++++++++++++++++++++
>  src/stat/fstatat.c | 27 +------------------------
>  src/stat/statx.c   | 35 +++++++++++++++++++++++++++++++++
>  3 files changed, 85 insertions(+), 26 deletions(-)
>  create mode 100644 src/stat/statx.c
> 
> diff --git a/include/sys/stat.h b/include/sys/stat.h
> index 10d446c4..81424462 100644
> --- a/include/sys/stat.h
> +++ b/include/sys/stat.h
> @@ -5,6 +5,7 @@ extern "C" {
>  #endif
>  
>  #include <features.h>
> +#include <stdint.h>

This can't be done unconditionally. Either the #include directive
needs to be within the preprocessor conditional where statx is, or the
individual __NEED_uint32_t etc need to be defined conditional on the
same condition before bits/alltypes.h is included. The latter is
probably better here.

>  #define __NEED_dev_t
>  #define __NEED_ino_t
> @@ -70,6 +71,54 @@ extern "C" {
>  #define UTIME_NOW  0x3fffffff
>  #define UTIME_OMIT 0x3ffffffe
>  
> +#if defined(_GNU_SOURCE)
> +#define STATX_TYPE 1U
> +#define STATX_MODE 2U
> +#define STATX_NLINK 4U
> +#define STATX_UID 8U
> +#define STATX_GID 0x10U
> +#define STATX_ATIME 0x20U
> +#define STATX_MTIME 0x40U
> +#define STATX_CTIME 0x80U
> +#define STATX_INO 0x100U
> +#define STATX_SIZE 0x200U
> +#define STATX_BLOCKS 0x400U
> +#define STATX_BASIC_STATS 0x7ffU
> +#define STATX_BTIME 0x800U
> +#define STATX_ALL 0xfffU
> +
> +struct statx_timestamp {
> +	int64_t tv_sec;
> +	uint32_t tv_nsec;
> +	int32_t __pad;
> +};

Minor nit but this could probably just be tv_nsec, __pad (both same
type). This also eliminates the gratuitous need to expose the signed
32-bit type which is not used elsewhere.

I was looking at whether *all* of the types here could be replaced
with equivalent ones that don't require exposing extra types, but the
ones documented as uint64_t probably can't. All the uint32_t in
principle could just be unsigned, and tv_sec time_t, but...

> +
> +struct statx {
> +	uint32_t stx_mask;
> +	uint32_t stx_blksize;
> +	uint64_t stx_attributes;
> +	uint32_t stx_nlink;
> +	uint32_t stx_uid;
> +	uint32_t stx_gid;
> +	uint16_t stx_mode;
> +	uint16_t __pad0[1];
> +	uint64_t stx_ino;
> +	uint64_t stx_size;
> +	uint64_t stx_blocks;
> +	uint64_t stx_attributes_mask;
> +	struct statx_timestamp stx_atime;
> +	struct statx_timestamp stx_btime;
> +	struct statx_timestamp stx_ctime;
> +	struct statx_timestamp stx_mtime;
> +	uint32_t stx_rdev_major;
> +	uint32_t stx_rdev_minor;
> +	uint32_t stx_dev_major;
> +	uint32_t stx_dev_minor;
> +	uint64_t __pad1[14];
> +};

stx_ino etc. should not be assuming ino_t etc. are defined the same as
uint64_t rather than something like unsigned long long. So we probably
just go with writing the types as documented...

> +
> +int statx(int, const char *__restrict, int, unsigned, struct statx *__restrict);
> +#endif
>  int stat(const char *__restrict, struct stat *__restrict);
>  int fstat(int, struct stat *);
>  int lstat(const char *__restrict, struct stat *__restrict);
> diff --git a/src/stat/fstatat.c b/src/stat/fstatat.c
> index 74c51cf5..5b2248a9 100644
> --- a/src/stat/fstatat.c
> +++ b/src/stat/fstatat.c
> @@ -7,36 +7,11 @@
>  #include <sys/sysmacros.h>
>  #include "syscall.h"
>  
> -struct statx {
> -	uint32_t stx_mask;
> -	uint32_t stx_blksize;
> -	uint64_t stx_attributes;
> -	uint32_t stx_nlink;
> -	uint32_t stx_uid;
> -	uint32_t stx_gid;
> -	uint16_t stx_mode;
> -	uint16_t pad1;
> -	uint64_t stx_ino;
> -	uint64_t stx_size;
> -	uint64_t stx_blocks;
> -	uint64_t stx_attributes_mask;
> -	struct {
> -		int64_t tv_sec;
> -		uint32_t tv_nsec;
> -		int32_t pad;
> -	} stx_atime, stx_btime, stx_ctime, stx_mtime;
> -	uint32_t stx_rdev_major;
> -	uint32_t stx_rdev_minor;
> -	uint32_t stx_dev_major;
> -	uint32_t stx_dev_minor;
> -	uint64_t spare[14];
> -};
> -
>  static int fstatat_statx(int fd, const char *restrict path, struct stat *restrict st, int flag)
>  {
>  	struct statx stx;
>  
> -	int ret = __syscall(SYS_statx, fd, path, flag, 0x7ff, &stx);
> +	int ret = __syscall(SYS_statx, fd, path, flag, STATX_BASIC_STATS, &stx);
>  	if (ret) return ret;
>  
>  	*st = (struct stat){

This can be a separate change from adding statx, but it's easy to
separate when merging.

> diff --git a/src/stat/statx.c b/src/stat/statx.c
> new file mode 100644
> index 00000000..ff49841b
> --- /dev/null
> +++ b/src/stat/statx.c
> @@ -0,0 +1,35 @@
> +#define _GNU_SOURCE
> +#include <sys/stat.h>
> +#include <syscall.h>
> +#include <sys/sysmacros.h>
> +#include <errno.h>
> +
> +int statx(int dirfd, const char *restrict path, int flags, unsigned mask, struct statx *restrict stx)
> +{
> +	int ret = syscall(SYS_statx, dirfd, path, flags, mask, stx);
> +	if (ret == ENOSYS) {
> +		struct stat st;
> +		fstatat(dir_fd, path, &st, flags);
> +		stx.stx_dev_major = major(st.st_dev);
> +		stx.stx_dev_minor = minor(st.st_dev);
> +		stx.stx_ino = st.st_ino;
> +		stx.stx_mode = st.st_mode;
> +		stx.stx_nlink = st.st_nlink;
> +		stx.stx_uid = st.st_uid;
> +		stx.stx_gid = st.st_gid;
> +		stx.stx_size = st.st_size;
> +		stx.stx_blksize = st.st_blksize;
> +		stx.stx_blocks = st.st_blocks;
> +		stx.stx_atime.tv_sec = st.st_atim.tv_sec;
> +		stx.stx_atime.tv_nsec = st.st_atim.tv_nsec;
> +		stx.stx_mtime.tv_sec = st.st_mtim.tv_sec;
> +		stx.stx_mtime.tv_nsec = st.st_mtim.tv_nsec;
> +		stx.stx_ctime.tv_sec = st.st_ctim.tv_sec;
> +		stx.stx_ctime.tv_nsec = st.st_ctim.tv_nsec;
> +		stx.stx_btime = 0;
> +		stx.stx_mask = STATX_BASIC_STATS;
> +		ret = EINVAL;
> +	}
> +
> +	return ret;
> +}
> -- 
> 2.34.1

I think this wasn't tested because it won't even compile (stx. instead
of stx->). It's also wrongly assuming syscall() returned a positive
error code rather than -1 on error and setting errno, but you should
be using the __syscall form that returns a negated error code, then
__syscall_ret at the end. I would write it as:

	int ret = __syscall(SYS_statx, dirfd, path, flags, mask, stx);
	if (ret != -ENOSYS) return __syscall_ret(ret);

then the fallback case outside a conditional. The fallback can't
assume fstatat succeeded like you're doing either. It needs to return
-1 immediately if fstatat fails.

All of this code needs to be conditional on SYS_fstatat existing, as
new archs don't have it and only have SYS_statx.

One annoying thing about this but I don't know a good fix; maybe you
have an idea: if SYS_statx fails with ENOSYS, the call to fstatat will
immediately perform SYS_statx again, only to have it fail, then
finally fall back to SYS_fstatat or other syscalls after two failures.
I'm not sure if it makes sense to expose __fstatat_kstat
libc-internally (hidden) to use here or do something else; that's
kinda getting into more complexity around this than I'd like for the
sake of optimizing old systems.

Rich