libbpf: Simplify BPF_CORE_READ_BITFIELD_PROBED usage

Streamline BPF_CORE_READ_BITFIELD_PROBED interface to follow
BPF_CORE_READ_BITFIELD (direct) and BPF_CORE_READ, in general, i.e., just
return read result or 0, if underlying bpf_probe_read() failed.

In practice, real applications rarely check bpf_probe_read() result, because
it has to always work or otherwise it's a bug. So propagating internal
bpf_probe_read() error from this macro hurts usability without providing real
benefits in practice. This patch fixes the issue and simplifies usage,
noticeable even in selftest itself.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191106201500.2582438-1-andriin@fb.com

tools/lib/bpf/bpf_core_read.h

@@ -39,32 +39,27 @@ enum bpf_field_info_kind {
 #endif
 
 /*
- * Extract bitfield, identified by src->field, and put its value into u64
- * *res. All this is done in relocatable manner, so bitfield changes such as
+ * Extract bitfield, identified by s->field, and return its value as u64.
+ * All this is done in relocatable manner, so bitfield changes such as
  * signedness, bit size, offset changes, this will be handled automatically.
  * This version of macro is using bpf_probe_read() to read underlying integer
  * storage. Macro functions as an expression and its return type is
  * bpf_probe_read()'s return value: 0, on success, <0 on error.
  */
-#define BPF_CORE_READ_BITFIELD_PROBED(src, field, res) ({		      \
-	unsigned long long val;						      \
+#define BPF_CORE_READ_BITFIELD_PROBED(s, field) ({			      \
+	unsigned long long val = 0;					      \
 									      \
-	*res = 0;							      \
-	val = __CORE_BITFIELD_PROBE_READ(res, src, field);		      \
-	if (!val) {							      \
-		*res <<= __CORE_RELO(src, field, LSHIFT_U64);		      \
-		val = __CORE_RELO(src, field, RSHIFT_U64);		      \
-		if (__CORE_RELO(src, field, SIGNED))			      \
-			*res = ((long long)*res) >> val;		      \
+	__CORE_BITFIELD_PROBE_READ(&val, s, field);			      \
+	val <<= __CORE_RELO(s, field, LSHIFT_U64);			      \
+	if (__CORE_RELO(s, field, SIGNED))				      \
+		val = ((long long)val) >> __CORE_RELO(s, field, RSHIFT_U64);  \
 	else								      \
-			*res = ((unsigned long long)*res) >> val;	      \
-		val = 0;						      \
-	}								      \
+		val = val >> __CORE_RELO(s, field, RSHIFT_U64);		      \
 	val;								      \
 })
 
 /*
- * Extract bitfield, identified by src->field, and return its value as u64.
+ * Extract bitfield, identified by s->field, and return its value as u64.
  * This version of macro is using direct memory reads and should be used from
  * BPF program types that support such functionality (e.g., typed raw
  * tracepoints).

tools/testing/selftests/bpf/progs/test_core_reloc_bitfields_probed.c

@@ -37,11 +37,6 @@ struct core_reloc_bitfields_output {
 	int64_t		s32;
 };
 
-#define TRANSFER_BITFIELD(in, out, field)				\
-	if (BPF_CORE_READ_BITFIELD_PROBED(in, field, &res))		\
-		return 1;						\
-	out->field = res
-
 SEC("raw_tracepoint/sys_enter")
 int test_core_bitfields(void *ctx)
 {
@@ -49,13 +44,13 @@ int test_core_bitfields(void *ctx)
 	struct core_reloc_bitfields_output *out = (void *)&data.out;
 	uint64_t res;
 
-	TRANSFER_BITFIELD(in, out, ub1);
-	TRANSFER_BITFIELD(in, out, ub2);
-	TRANSFER_BITFIELD(in, out, ub7);
-	TRANSFER_BITFIELD(in, out, sb4);
-	TRANSFER_BITFIELD(in, out, sb20);
-	TRANSFER_BITFIELD(in, out, u32);
-	TRANSFER_BITFIELD(in, out, s32);
+	out->ub1 = BPF_CORE_READ_BITFIELD_PROBED(in, ub1);
+	out->ub2 = BPF_CORE_READ_BITFIELD_PROBED(in, ub2);
+	out->ub7 = BPF_CORE_READ_BITFIELD_PROBED(in, ub7);
+	out->sb4 = BPF_CORE_READ_BITFIELD_PROBED(in, sb4);
+	out->sb20 = BPF_CORE_READ_BITFIELD_PROBED(in, sb20);
+	out->u32 = BPF_CORE_READ_BITFIELD_PROBED(in, u32);
+	out->s32 = BPF_CORE_READ_BITFIELD_PROBED(in, s32);
 
 	return 0;
 }