[LTP] [PATCH v3 1/2] userfaultfd: Add helper for checking UFFD feature support

[LTP] [PATCH v3 1/2] userfaultfd: Add helper for checking UFFD feature support

[Ricardo Branco]

Add CHECK_UFFD_FEATURE() to perform the initial UFFDIO_API handshake
without requesting optional features and verify that the kernel reports
the requested feature bit.

Signed-off-by: Ricardo Branco <rbranco@suse.de>
---
 include/lapi/userfaultfd.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/lapi/userfaultfd.h b/include/lapi/userfaultfd.h
index aab3890b7..170dfadd4 100644
--- a/include/lapi/userfaultfd.h
+++ b/include/lapi/userfaultfd.h
@@ -277,4 +277,24 @@ retry:
 	return ret;
 }
 
+#define CHECK_UFFD_FEATURE(feature)	check_uffd_feature(feature, #feature)
+
+static inline void check_uffd_feature(uint64_t feature, const char *name)
+{
+	struct uffdio_api uffdio_api = {};
+	int uffd;
+
+	uffd = SAFE_USERFAULTFD(O_CLOEXEC | O_NONBLOCK, false);
+
+	uffdio_api.api = UFFD_API;
+	SAFE_IOCTL(uffd, UFFDIO_API, &uffdio_api);
+
+	if (!(uffdio_api.features & feature)) {
+		SAFE_CLOSE(uffd);
+		tst_brk(TCONF, "%s not supported", name);
+	}
+
+	SAFE_CLOSE(uffd);
+}
+
 #endif /* LAPI_USERFAULTFD_H__ */
-- 
2.54.0


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[LTP] [PATCH v3 2/2] userfaultfd: Use two-step handshake to probe features

[Ricardo Branco]

userfaultfd05 and userfaultfd06 assume that
UFFD_FEATURE_PAGEFAULT_FLAG_WP and UFFD_FEATURE_POISON are available
when setting up userfaultfd.

On kernels where either feature is unavailable at runtime,
UFFDIO_API fails with EINVAL and the tests end up as TBROK instead of
being skipped.

Fix this by using the required two-step handshake and skip with TCONF
if the required feature bit is absent.

Fixes: 1840ee23d172b5ab04cca7c2acfa48755b041911
Link: https://github.com/linux-test-project/ltp/issues/1289

Signed-off-by: Ricardo Branco <rbranco@suse.de>
---
 testcases/kernel/syscalls/userfaultfd/userfaultfd05.c | 4 +++-
 testcases/kernel/syscalls/userfaultfd/userfaultfd06.c | 6 +++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/testcases/kernel/syscalls/userfaultfd/userfaultfd05.c b/testcases/kernel/syscalls/userfaultfd/userfaultfd05.c
index 59956d85d..55ddc1a69 100644
--- a/testcases/kernel/syscalls/userfaultfd/userfaultfd05.c
+++ b/testcases/kernel/syscalls/userfaultfd/userfaultfd05.c
@@ -87,10 +87,12 @@ static void *handle_thread(void *arg LTP_ATTRIBUTE_UNUSED)
 static void run(void)
 {
 	pthread_t thr;
-	struct uffdio_api uffdio_api;
+	struct uffdio_api uffdio_api = {};
 	struct uffdio_register uffdio_register;
 	struct uffdio_writeprotect uffdio_writeprotect;
 
+	CHECK_UFFD_FEATURE(UFFD_FEATURE_PAGEFAULT_FLAG_WP);
+
 	set_pages();
 
 	uffd = SAFE_USERFAULTFD(O_CLOEXEC | O_NONBLOCK, false);
diff --git a/testcases/kernel/syscalls/userfaultfd/userfaultfd06.c b/testcases/kernel/syscalls/userfaultfd/userfaultfd06.c
index ec93d8ad6..c06de4ca2 100644
--- a/testcases/kernel/syscalls/userfaultfd/userfaultfd06.c
+++ b/testcases/kernel/syscalls/userfaultfd/userfaultfd06.c
@@ -99,6 +99,9 @@ static void run(void)
 
 	poison_fault_seen = 0;
 	sigbus_seen = 0;
+
+	CHECK_UFFD_FEATURE(UFFD_FEATURE_POISON);
+
 	set_pages();
 
 	uffd = SAFE_USERFAULTFD(O_CLOEXEC | O_NONBLOCK, false);
@@ -108,9 +111,6 @@ static void run(void)
 
 	SAFE_IOCTL(uffd, UFFDIO_API, &uffdio_api);
 
-	if (!(uffdio_api.features & UFFD_FEATURE_POISON))
-		tst_brk(TCONF, "UFFD_FEATURE_POISON not supported");
-
 	uffdio_register.range.start = (unsigned long) page;
 	uffdio_register.range.len = page_size;
 	uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
-- 
2.54.0


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Re: [LTP] [PATCH v3 2/2] userfaultfd: Use two-step handshake to probe features

[Cyril Hrubis]

Hi!
>  static void run(void)
>  {
>  	pthread_t thr;
> -	struct uffdio_api uffdio_api;
> +	struct uffdio_api uffdio_api = {};
>  	struct uffdio_register uffdio_register;
>  	struct uffdio_writeprotect uffdio_writeprotect;
>  
> +	CHECK_UFFD_FEATURE(UFFD_FEATURE_PAGEFAULT_FLAG_WP);

Can we please move this into the test setup() so that it's executed only
once if the test is started with -i parameter?


-- 
Cyril Hrubis
chrubis@suse.cz

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Re: [LTP] [PATCH v3 1/2] userfaultfd: Add helper for checking UFFD feature support

[Cyril Hrubis]

Hi!
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>

-- 
Cyril Hrubis
chrubis@suse.cz

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Re: [LTP] userfaultfd: Add helper for checking UFFD feature support

[linuxtestproject.agent]

Hi Ricardo,

--- [PATCH 1/2] ---

On Thu, 7 May 2026, Ricardo Branco wrote:
> userfaultfd: Add helper for checking UFFD feature support
>
> Add CHECK_UFFD_FEATURE() to perform the initial UFFDIO_API handshake
> without requesting optional features and verify that the kernel reports
> the requested feature bit.

The body only explains the mechanism, not the motivation. Add a sentence
explaining why this helper is needed — e.g. "Without this two-step probe,
requesting an unsupported feature in UFFDIO_API fails with EINVAL,
causing tests to TBROK instead of TCONF."

--- [PATCH 2/2] ---

On Thu, 7 May 2026, Ricardo Branco wrote:
> Fixes: 1840ee23d172b5ab04cca7c2acfa48755b041911

Please verify this SHA resolves in the full LTP history; it is not
reachable in this clone.

Pre-existing issues noticed in the surrounding code (not introduced
by this patch):

- testcases/kernel/syscalls/userfaultfd/userfaultfd05.c — `wp_fault_seen`
  is never reset to 0 at the top of run(); on multi-iteration runs (-i N)
  the second iteration may falsely pass.

---
Note:

Our agent completed the review of the patch.

The agent can sometimes produce false positives although often its
findings are genuine. If you find issues with the review, please
comment this email or ignore the suggestions.

Regards,
LTP AI Reviewer

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