[PATCH RFC 2/3] libfile: implement read_fd counterpart to read_file

[Sascha Hauer]

On Mon, Nov 20, 2023 at 09:37:49AM +0100, Ahmad Fatoum wrote:
> Files opened with O_TMPFILE have no name, so read_file can't be used
> with them. Therefore add a read_fd function, which slurps all a file's
> contents into a buffer.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
> ---
>  include/libfile.h |  2 ++
>  lib/libfile.c     | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/include/libfile.h b/include/libfile.h
> index a353ccfa9ea9..423e7ffec5b7 100644
> --- a/include/libfile.h
> +++ b/include/libfile.h
> @@ -12,6 +12,8 @@ char *read_file_line(const char *fmt, ...);
>  
>  void *read_file(const char *filename, size_t *size);
>  
> +void *read_fd(int fd, size_t *size);
> +
>  int read_file_2(const char *filename, size_t *size, void **outbuf,
>  		loff_t max_size);
>  
> diff --git a/lib/libfile.c b/lib/libfile.c
> index e53ba08415a2..c257baaa2733 100644
> --- a/lib/libfile.c
> +++ b/lib/libfile.c
> @@ -306,6 +306,50 @@ void *read_file(const char *filename, size_t *size)
>  }
>  EXPORT_SYMBOL(read_file);
>  
> +/**
> + * read_fd - read from a file descriptor to an allocated buffer
> + * @filename:  The file descriptor to read
> + * @size:      After successful return contains the size of the file
> + *
> + * This function reads a file descriptor from offset 0 until EOF to an
> + * allocated buffer.
> + *
> + * Return: The buffer containing the file or NULL on failure
> + */
> +void *read_fd(int fd, size_t *out_size)
> +{
> +	off_t off;
> +	ssize_t ret;
> +	size_t size;
> +	void *buf;
> +
> +	off = lseek(fd, SEEK_CUR, 0);

You lseek to the current position. It seems you are trying to determine
the size of the file, so did you mean to use SEEK_END?

You could use fstat() instead. With that you could also correctly handle
FILE_SIZE_STREAM.

> +	if (off >= 0) {
> +		size = off;
> +		off = lseek(fd, SEEK_SET, 0);
> +	}
> +	if (off < 0) {
> +		ret = off;
> +		goto close_fd;
> +	}
> +
> +	buf = malloc(size + 3);
> +	ret = read_full(fd, buf, size);

You can use pread_full() here to avoid having to lseek to the beginning
of the file.

> +	if (ret < 0) {
> +		free(buf);
> +		goto close_fd;
> +	}
> +
> +	memset(&buf[size], '\0', 3);
> +	*out_size = size;
> +
> +close_fd:
> +	close(fd);
> +
> +	return ret < 0 ? NULL : buf;

Return an error pointer or an error integer and pass buf as argument?

Sascha

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