[PATCH v2 0/2] nolibc: Add dirfd()

[PATCH v2 0/2] nolibc: Add dirfd()

[Daniel Palmer]

Add dirfd() to convert a DIR back to an fd for functions like fstatat()
that take an fd.

Change log:
v2 - Fix the name of the dirfd helper, I debated over removing the helper
     as suggested by Thomas but it seems right to have the logic in a single
     place.
     Add test a basic test to make sure dirfd() actually does convert back
     to the expected fd and using dirfd() with fstatat().

Daniel Palmer (2):
  tools/nolibc: Add dirfd()
  selftests/nolibc: Add test for dirfd()

 tools/include/nolibc/dirent.h                | 38 ++++++++++++----
 tools/testing/selftests/nolibc/nolibc-test.c | 47 ++++++++++++++++++++
 2 files changed, 77 insertions(+), 8 deletions(-)

-- 
2.53.0

[PATCH v2 1/2] tools/nolibc: Add dirfd()

[Daniel Palmer]

This is useful for things like fstatat() on children of a directory
opened with opendir().

At the same time use the new internal helper to replace the two places
that are converting the DIR pointer back into an fd.

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
---
 tools/include/nolibc/dirent.h | 38 +++++++++++++++++++++++++++--------
 1 file changed, 30 insertions(+), 8 deletions(-)

diff --git a/tools/include/nolibc/dirent.h b/tools/include/nolibc/dirent.h
index 4e02ef25e72d..d282ea8eb5d3 100644
--- a/tools/include/nolibc/dirent.h
+++ b/tools/include/nolibc/dirent.h
@@ -27,6 +27,18 @@ typedef struct {
 	char dummy[1];
 } DIR;
 
+/* Internal dirfd() that does not set errno */
+static __attribute__((unused))
+int __nolibc_dirfd(DIR *dirp)
+{
+	intptr_t i = (intptr_t)dirp;
+
+	if (i >= 0)
+		return -1;
+
+	return ~i;
+}
+
 static __attribute__((unused))
 DIR *fdopendir(int fd)
 {
@@ -49,15 +61,27 @@ DIR *opendir(const char *name)
 }
 
 static __attribute__((unused))
-int closedir(DIR *dirp)
+int dirfd(DIR *dirp)
 {
-	intptr_t i = (intptr_t)dirp;
+	int fd = __nolibc_dirfd(dirp);
 
-	if (i >= 0) {
+	if (fd < 0) {
 		SET_ERRNO(EBADF);
 		return -1;
 	}
-	return close(~i);
+
+	return fd;
+}
+
+static __attribute__((unused))
+int closedir(DIR *dirp)
+{
+	int fd = dirfd(dirp);
+
+	if (fd < 0)
+		return -1;
+
+	return close(fd);
 }
 
 static __attribute__((unused))
@@ -65,14 +89,12 @@ int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result)
 {
 	char buf[sizeof(struct linux_dirent64) + NAME_MAX + 1] __nolibc_aligned_as(struct linux_dirent64);
 	struct linux_dirent64 *ldir = (void *)buf;
-	intptr_t i = (intptr_t)dirp;
 	int fd, ret;
 
-	if (i >= 0)
+	fd = __nolibc_dirfd(dirp);
+	if (fd < 0)
 		return EBADF;
 
-	fd = ~i;
-
 	ret = _sys_getdents64(fd, ldir, sizeof(buf));
 	if (ret < 0)
 		return -ret;
-- 
2.53.0

Re: [PATCH v2 1/2] tools/nolibc: Add dirfd()

[Thomas Weißschuh]

On 2026-10-07 14:29:12+0900, Daniel Palmer wrote:
> This is useful for things like fstatat() on children of a directory
> opened with opendir().
> 
> At the same time use the new internal helper to replace the two places
> that are converting the DIR pointer back into an fd.
> 
> Signed-off-by: Daniel Palmer <daniel@thingy.jp>
> ---
>  tools/include/nolibc/dirent.h | 38 +++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/include/nolibc/dirent.h b/tools/include/nolibc/dirent.h
> index 4e02ef25e72d..d282ea8eb5d3 100644
> --- a/tools/include/nolibc/dirent.h
> +++ b/tools/include/nolibc/dirent.h
> @@ -27,6 +27,18 @@ typedef struct {
>  	char dummy[1];
>  } DIR;
>  
> +/* Internal dirfd() that does not set errno */
> +static __attribute__((unused))
> +int __nolibc_dirfd(DIR *dirp)
> +{
> +	intptr_t i = (intptr_t)dirp;
> +
> +	if (i >= 0)
> +		return -1;
> +
> +	return ~i;
> +}

I'm fine with keeping __nolibc_dirfd(), but the diffs would be nicer if
it was introduced in a dedicated commit.

> +
>  static __attribute__((unused))
>  DIR *fdopendir(int fd)
>  {
> @@ -49,15 +61,27 @@ DIR *opendir(const char *name)
>  }
>  
>  static __attribute__((unused))
> -int closedir(DIR *dirp)
> +int dirfd(DIR *dirp)
>  {
> -	intptr_t i = (intptr_t)dirp;
> +	int fd = __nolibc_dirfd(dirp);
>  
> -	if (i >= 0) {
> +	if (fd < 0) {
>  		SET_ERRNO(EBADF);
>  		return -1;
>  	}
> -	return close(~i);
> +
> +	return fd;
> +}

(...)

[PATCH v2 2/2] selftests/nolibc: Add test for dirfd()

[Daniel Palmer]

Test that open() and dirfd() use the same fd and then use
dirfd() with fstatat() to stat each of its children.

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
---
 tools/testing/selftests/nolibc/nolibc-test.c | 47 ++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index ed860b0a15a1..82616f2c1ba4 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -858,6 +858,52 @@ static int test_dirent(void)
 	return 0;
 }
 
+static int test_dirfd(void)
+{
+	struct dirent dirent, *result;
+	int ret = 0, fd;
+	struct stat st;
+	DIR *dir;
+
+	fd = open("/", O_RDONLY | O_DIRECTORY);
+	if (fd < 0)
+		return __LINE__;
+
+	dir = fdopendir(fd);
+	if (!dir) {
+		close(fd);
+		return __LINE__;
+	}
+
+	/* Check that dirfd() gives the same fd as open() */
+	if (dirfd(dir) != fd) {
+		ret = __LINE__;
+		goto out;
+	}
+
+	/* Loop through the children of the directory calling fstatat() on each */
+	while (1) {
+		if (readdir_r(dir, &dirent, &result) != 0) {
+			ret = __LINE__;
+			goto out;
+		}
+
+		if (!result)
+			break;
+
+		if (fstatat(dirfd(dir), dirent.d_name, &st, AT_SYMLINK_NOFOLLOW) != 0) {
+			ret = __LINE__;
+			goto out;
+
+		}
+	}
+
+out:
+	closedir(dir);
+
+	return ret;
+}
+
 int test_getcwd(void)
 {
 	char cwd_syscall[PATH_MAX];
@@ -1651,6 +1697,7 @@ int run_syscall(int min, int max)
 		CASE_TEST(getdents64_root);   EXPECT_SYSNE(1, test_getdents64("/"), -1); break;
 		CASE_TEST(getdents64_null);   EXPECT_SYSER(1, test_getdents64("/dev/null"), -1, ENOTDIR); break;
 		CASE_TEST(directories);       EXPECT_SYSZR(is_nolibc && proc, test_dirent()); break;
+		CASE_TEST(dirfd);             EXPECT_SYSZR(is_nolibc, test_dirfd()); break;
 		CASE_TEST(getrandom);         EXPECT_SYSZR(1, test_getrandom()); break;
 		CASE_TEST(gettimeofday_tv);   EXPECT_SYSZR(1, gettimeofday(&tv, NULL)); break;
 		CASE_TEST(gettimeofday_tv_tz);EXPECT_SYSZR(1, gettimeofday(&tv, &tz)); break;
-- 
2.53.0

Re: [PATCH v2 2/2] selftests/nolibc: Add test for dirfd()

[Thomas Weißschuh]

Hi Daniel,

On 2026-10-07 14:29:13+0900, Daniel Palmer wrote:
> Test that open() and dirfd() use the same fd and then use
> dirfd() with fstatat() to stat each of its children.

In general the series looks good. I have some minor requests,
but can also implement these myself if you prefer.
Also I'd like to push the patches into the next development cycles.

> Signed-off-by: Daniel Palmer <daniel@thingy.jp>
> ---
>  tools/testing/selftests/nolibc/nolibc-test.c | 47 ++++++++++++++++++++
>  1 file changed, 47 insertions(+)
> 
> diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> index ed860b0a15a1..82616f2c1ba4 100644
> --- a/tools/testing/selftests/nolibc/nolibc-test.c
> +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> @@ -858,6 +858,52 @@ static int test_dirent(void)
>  	return 0;
>  }
>  
> +static int test_dirfd(void)
> +{
> +	struct dirent dirent, *result;
> +	int ret = 0, fd;
> +	struct stat st;
> +	DIR *dir;
> +
> +	fd = open("/", O_RDONLY | O_DIRECTORY);
> +	if (fd < 0)
> +		return __LINE__;
> +
> +	dir = fdopendir(fd);
> +	if (!dir) {
> +		close(fd);
> +		return __LINE__;
> +	}
> +
> +	/* Check that dirfd() gives the same fd as open() */
> +	if (dirfd(dir) != fd) {
> +		ret = __LINE__;
> +		goto out;
> +	}
> +
> +	/* Loop through the children of the directory calling fstatat() on each */

What is the advantage of reading all entries here?
We already do something similar in test_dirent().
Reading a single entry should be enough.

> +	while (1) {
> +		if (readdir_r(dir, &dirent, &result) != 0) {
> +			ret = __LINE__;
> +			goto out;
> +		}
> +
> +		if (!result)
> +			break;
> +
> +		if (fstatat(dirfd(dir), dirent.d_name, &st, AT_SYMLINK_NOFOLLOW) != 0) {
> +			ret = __LINE__;
> +			goto out;
> +
> +		}
> +	}
> +
> +out:
> +	closedir(dir);
> +
> +	return ret;
> +}
> +
>  int test_getcwd(void)
>  {
>  	char cwd_syscall[PATH_MAX];
> @@ -1651,6 +1697,7 @@ int run_syscall(int min, int max)
>  		CASE_TEST(getdents64_root);   EXPECT_SYSNE(1, test_getdents64("/"), -1); break;
>  		CASE_TEST(getdents64_null);   EXPECT_SYSER(1, test_getdents64("/dev/null"), -1, ENOTDIR); break;
>  		CASE_TEST(directories);       EXPECT_SYSZR(is_nolibc && proc, test_dirent()); break;
> +		CASE_TEST(dirfd);             EXPECT_SYSZR(is_nolibc, test_dirfd()); break;

Could you mention in the commit message why this is specific to nolibc?

>  		CASE_TEST(getrandom);         EXPECT_SYSZR(1, test_getrandom()); break;
>  		CASE_TEST(gettimeofday_tv);   EXPECT_SYSZR(1, gettimeofday(&tv, NULL)); break;
>  		CASE_TEST(gettimeofday_tv_tz);EXPECT_SYSZR(1, gettimeofday(&tv, &tz)); break;
> -- 
> 2.53.0
> 

Re: [PATCH v2 0/2] nolibc: Add dirfd()

[Willy Tarreau]

Hi Daniel,

On Wed, Oct 07, 2026 at 02:29:11PM +0900, Daniel Palmer wrote:
> Add dirfd() to convert a DIR back to an fd for functions like fstatat()
> that take an fd.
> 
> Change log:
> v2 - Fix the name of the dirfd helper, I debated over removing the helper
>      as suggested by Thomas but it seems right to have the logic in a single
>      place.
>      Add test a basic test to make sure dirfd() actually does convert back
>      to the expected fd and using dirfd() with fstatat().

The series looks good to me:

  Reviewed-by: Willy Tarreau <w@1wt.eu>

Let's wait for Thomas in case he spots anything subtle.

Thanks!
Willy