[LTP] syscalls/cachestat05: Add test for permission checks

[linuxtestproject.agent@gmail.com]

Hi Xuewen,

On Thu Nov 5 18:46:17 2026 +0800, Xuewen Wang <wangxuewen@kylinos.cn> wrote:
> syscalls/cachestat05: Add test for permission checks

> +/*\
> + * Verify that cachestat() is only allowed on file descriptors with write
> + * access to the underlying file.

Use :manpage:`cachestat(2)` when referring to the syscall in the
documentation comment block.

> +static struct tcase {
> +	const char *file;
> +	int open_flags;
> +	int exp_errno;
> +	int needs_fix;
> +	const char *desc;
> +} tcases[] = {
> +	{FILE_RDWR, O_RDWR, 0, 0, "descriptor open for writing"},
> +	{FILE_OWNED, O_RDONLY, 0, 0, "file owned by the caller"},
> +	{FILE_RDWR, O_RDONLY, 0, 0, "file writable by the caller"},
> +	{FILE_RDONLY, O_RDONLY, EPERM, 1, "file read-only for the caller"},
> +};
> +
> +static void verify_cachestat(unsigned int n)
> +{
> +	struct tcase *tc = &tcases[n];
> +	int fd;
> +
> +	if (tc->needs_fix && tst_kvercmp(6, 14, 0) < 0) {
> +		tst_res(TCONF, "cachestat() permission check added in v6.14");
> +		return;
> +	}

Remove the kernel version check and tc->needs_fix. Commit 5f537664e705 is a
bug fix already recorded in .tags, so unpatched kernels should fail and print
the missing-fix hint rather than skipping with TCONF.

> +	SAFE_SETGROUPS(0, NULL);
> +	SAFE_SETGID(gid);
> +	SAFE_SETUID(uid);
> +
> +	fd = SAFE_OPEN(tc->file, tc->open_flags);

Opening FILE_RDWR (mode 0666) after dropping privileges means the caller also
has write permission on the file, so the FMODE_WRITE check in
can_do_cachestat() is not isolated from file_permission(f, MAY_WRITE). Open
tc->file before dropping privileges and use FILE_RDONLY with O_RDWR for the
first test case.

> +	fd = SAFE_OPEN(FILE_OWNED, O_CREAT | O_RDWR, 0600);
> +	SAFE_CLOSE(fd);
> +	SAFE_CHOWN(FILE_OWNED, uid, -1);

FILE_OWNED is created with mode 0600, which gives the owner write permission
and satisfies file_permission(f, MAY_WRITE) in addition to
file_owner_or_capable(). Create FILE_OWNED with mode 0400 so the ownership
check is tested without write permission.

Verdict - Needs revision

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