[PATCH bpf-next v4 3/8] selftests/bpf: make arena_base() opaque to GCC's -Warray-bounds

[Vineet Gupta <vineet.gupta@linux.dev>]

arena_base() casts the map definition to the kernel's struct bpf_arena
and reads user_vm_start out of it. In the object file that definition
is only the 32-byte BTF placeholder the loader rewrites, so the access
is past the end of the object which GCC correctly complaints about and
due to -Werror leads to build failure:

  libarena/include/bpf_arena_common.h:52:69: error: array subscript 'struct bpf_arena[0]' is partly outside array bounds of 'struct <anonymous>[1]' [-Werror=array-bounds=]
     52 | #define arena_base(map) ((void __arena *)((struct bpf_arena *)(map))->user_vm_start)
  progs/arena_race.c:58:30: note: in expansion of macro 'arena_base'

progs/stream.c already solved this for its own open-coded copy of the
access, with barrier_var(), for both the compilers. Do the same in
arena_base() itself so all such uses are fixed.

No test changes verdict; this only lets the objects build.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
---
v3 -> v4: was 4/8. barrier_var() for both compilers rather than
behind #ifdef __clang__, and stream.c's five open-coded copies of the
access fold back into the macro. (Alexei)

 .../bpf/libarena/include/bpf_arena_common.h   | 11 +++-
 tools/testing/selftests/bpf/progs/stream.c    | 56 +++----------------
 2 files changed, 18 insertions(+), 49 deletions(-)

diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
index 82aafe879fae..0eba52961405 100644
--- a/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
+++ b/tools/testing/selftests/bpf/libarena/include/bpf_arena_common.h
@@ -49,7 +49,16 @@ void __arena* bpf_arena_alloc_pages(void *map, void __arena *addr, __u32 page_cn
 int bpf_arena_reserve_pages(void *map, void __arena *addr, __u32 page_cnt) __ksym __weak;
 void bpf_arena_free_pages(void *map, void __arena *ptr, __u32 page_cnt) __ksym __weak;
 
-#define arena_base(map) ((void __arena *)((struct bpf_arena *)(map))->user_vm_start)
+/*
+ * The map definition is smaller than struct bpf_arena.
+ * barrier_var() hides that from -Warray-bounds of GCC.
+ */
+#define arena_base(map)							\
+	({								\
+		struct bpf_arena *___arena = (void *)(map);		\
+		barrier_var(___arena);					\
+		(void __arena *)___arena->user_vm_start;		\
+	})
 
 #else /* when compiled as user space code */
 
diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
index 1ae015c4e6b4..b4d88d0e01ad 100644
--- a/tools/testing/selftests/bpf/progs/stream.c
+++ b/tools/testing/selftests/bpf/progs/stream.c
@@ -178,16 +178,8 @@ __stderr("Call trace:\n"
 "|[ \t]+[^\n]+\n)*}}")
 int stream_arena_write_fault(void *ctx)
 {
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start;
+	u64 user_vm_start = (u64)arena_base(&arena);
 
-	/* Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than struct
-	 * bpf_arena. barrier_var() makes the pointer opaque to GCC, preventing the
-	 * bounds analysis
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	asm volatile (
@@ -212,16 +204,8 @@ __stderr("Call trace:\n"
 "|[ \t]+[^\n]+\n)*}}")
 int stream_arena_read_fault(void *ctx)
 {
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start;
+	u64 user_vm_start = (u64)arena_base(&arena);
 
-	/* Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than struct
-	 * bpf_arena. barrier_var() makes the pointer opaque to GCC, preventing the
-	 * bounds analysis
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	asm volatile (
@@ -252,17 +236,9 @@ int stream_arena_load_acquire_fault(void *ctx)
 		.off	 = 0x7fff,
 		.imm	 = 0x100,	/* BPF_LOAD_ACQ */
 	};
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start, val;
+	u64 user_vm_start = (u64)arena_base(&arena);
+	u64 val;
 
-	/*
-	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than
-	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
-	 * preventing the bounds analysis.
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	asm volatile (
@@ -296,17 +272,9 @@ int stream_arena_xchg_fault(void *ctx)
 		.off	 = 0x7fff,
 		.imm	 = 0xe1,	/* BPF_XCHG */
 	};
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start, val;
+	u64 user_vm_start = (u64)arena_base(&arena);
+	u64 val;
 
-	/*
-	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than
-	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
-	 * preventing the bounds analysis.
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	/*
@@ -346,17 +314,9 @@ int stream_arena_cmpxchg_fault(void *ctx)
 		.off	 = 0x7fff,
 		.imm	 = 0xf1,	/* BPF_CMPXCHG */
 	};
-	struct bpf_arena *ptr = (void *)&arena;
-	u64 user_vm_start, val;
+	u64 user_vm_start = (u64)arena_base(&arena);
+	u64 val;
 
-	/*
-	 * Prevent GCC bounds warning: casting &arena to struct bpf_arena *
-	 * triggers bounds checking since the map definition is smaller than
-	 * struct bpf_arena. barrier_var() makes the pointer opaque to GCC,
-	 * preventing the bounds analysis.
-	 */
-	barrier_var(ptr);
-	user_vm_start = ptr->user_vm_start;
 	fault_addr = user_vm_start + 0x7fff;
 	bpf_addr_space_cast(user_vm_start, 0, 1);
 	/*
-- 
2.53.0-Meta