selftests/bpf: Test rhtab kptr cancellation semanticsgrafted

Resizable hash-map updates and deletions must not perform full special-field
destruction in their caller context. In particular, a referenced kptr must
remain attached to the allocation until the memory allocator destructor can
release it safely.

Add separate coverage for both affected paths. The update test stores a task
kptr, replaces the ordinary value bytes with BPF_EXIST, and verifies that the
kptr survived. The delete test removes an element and exchanges its kptr
through the still-valid map-value pointer before the allocation is reclaimed.

Both cases observe a NULL kptr when rhtab uses bpf_obj_free_fields(). They
recover and release the reference after rhtab switches to cancellation
semantics.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
[ kkd: Split update and delete coverage and rewrote the commit log ]
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904104203.345917-5-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 87 insertions, 0 deletions

diff --git a/rust/helpers/atomic_ext.c b/rust/helpers/atomic_ext.c
new file mode 100644
index 000000000..c267d5190
--- /dev/null
+++ b/rust/helpers/atomic_ext.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <asm/barrier.h>
+#include <asm/rwonce.h>
+#include <linux/atomic.h>
+
+#define GEN_READ_HELPER(tname, type)						\
+__rust_helper type rust_helper_atomic_##tname##_read(type *ptr)			\
+{										\
+	return READ_ONCE(*ptr);							\
+}
+
+#define GEN_SET_HELPER(tname, type)						\
+__rust_helper void rust_helper_atomic_##tname##_set(type *ptr, type val)	\
+{										\
+	WRITE_ONCE(*ptr, val);							\
+}
+
+#define GEN_READ_ACQUIRE_HELPER(tname, type)					\
+__rust_helper type rust_helper_atomic_##tname##_read_acquire(type *ptr)		\
+{										\
+	return smp_load_acquire(ptr);						\
+}
+
+#define GEN_SET_RELEASE_HELPER(tname, type)					\
+__rust_helper void rust_helper_atomic_##tname##_set_release(type *ptr, type val)\
+{										\
+	smp_store_release(ptr, val);						\
+}
+
+#define GEN_READ_SET_HELPERS(tname, type)					\
+	GEN_READ_HELPER(tname, type)						\
+	GEN_SET_HELPER(tname, type)						\
+	GEN_READ_ACQUIRE_HELPER(tname, type)					\
+	GEN_SET_RELEASE_HELPER(tname, type)					\
+
+GEN_READ_SET_HELPERS(i8, s8)
+GEN_READ_SET_HELPERS(i16, s16)
+GEN_READ_SET_HELPERS(ptr, const void *)
+
+/*
+ * xchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the
+ * architecture provding xchg() support for i8 and i16.
+ *
+ * The architectures that currently support Rust (x86_64, armv7,
+ * arm64, riscv, and loongarch) satisfy these requirements.
+ */
+#define GEN_XCHG_HELPER(tname, type, suffix)					\
+__rust_helper type								\
+rust_helper_atomic_##tname##_xchg##suffix(type *ptr, type new)			\
+{										\
+	return xchg##suffix(ptr, new);					\
+}
+
+#define GEN_XCHG_HELPERS(tname, type)						\
+	GEN_XCHG_HELPER(tname, type, )						\
+	GEN_XCHG_HELPER(tname, type, _acquire)					\
+	GEN_XCHG_HELPER(tname, type, _release)					\
+	GEN_XCHG_HELPER(tname, type, _relaxed)					\
+
+GEN_XCHG_HELPERS(i8, s8)
+GEN_XCHG_HELPERS(i16, s16)
+GEN_XCHG_HELPERS(ptr, const void *)
+
+/*
+ * try_cmpxchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the
+ * architecture provding try_cmpxchg() support for i8 and i16.
+ *
+ * The architectures that currently support Rust (x86_64, armv7,
+ * arm64, riscv, and loongarch) satisfy these requirements.
+ */
+#define GEN_TRY_CMPXCHG_HELPER(tname, type, suffix)				\
+__rust_helper bool								\
+rust_helper_atomic_##tname##_try_cmpxchg##suffix(type *ptr, type *old, type new)\
+{										\
+	return try_cmpxchg##suffix(ptr, old, new);				\
+}
+
+#define GEN_TRY_CMPXCHG_HELPERS(tname, type)					\
+	GEN_TRY_CMPXCHG_HELPER(tname, type, )					\
+	GEN_TRY_CMPXCHG_HELPER(tname, type, _acquire)				\
+	GEN_TRY_CMPXCHG_HELPER(tname, type, _release)				\
+	GEN_TRY_CMPXCHG_HELPER(tname, type, _relaxed)				\
+
+GEN_TRY_CMPXCHG_HELPERS(i8, s8)
+GEN_TRY_CMPXCHG_HELPERS(i16, s16)
+GEN_TRY_CMPXCHG_HELPERS(ptr, const void *)