bpf: fix uninitialized values in BPF_{CORE,PROBE}_READ

With the latest LLVM bpf selftests build will fail with
the following error message:

    progs/profiler.inc.h:710:31: error: default initialization of an object of type 'typeof ((parent_task)->real_cred->uid.val)' (aka 'const unsigned int') leaves the object uninitialized and is incompatible with C++ [-Werror,-Wdefault-const-init-unsafe]
      710 |         proc_exec_data->parent_uid = BPF_CORE_READ(parent_task, real_cred, uid.val);
          |                                      ^
    tools/testing/selftests/bpf/tools/include/bpf/bpf_core_read.h:520:35: note: expanded from macro 'BPF_CORE_READ'
      520 |         ___type((src), a, ##__VA_ARGS__) __r;                               \
          |                                          ^

Fix this by declaring __r to be an array of __u8 of a proper size.

Fixes: 792001f ("libbpf: Add user-space variants of BPF_CORE_READ() family of macros")
Fixes: a4b09a9 ("libbpf: Add non-CO-RE variants of BPF_CORE_READ() macro family")
Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>

Author: aspsk

tools/lib/bpf/bpf_core_read.h

@@ -517,9 +517,9 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
  * than enough for any practical purpose.
  */
 #define BPF_CORE_READ(src, a, ...) ({					    \
-	___type((src), a, ##__VA_ARGS__) __r;				    \
+	__u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))];		    \
 	BPF_CORE_READ_INTO(&__r, (src), a, ##__VA_ARGS__);		    \
-	__r;								    \
+	*(___type((src), a, ##__VA_ARGS__) *)__r;			    \
 })
 
 /*
@@ -533,16 +533,16 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
  * input argument.
  */
 #define BPF_CORE_READ_USER(src, a, ...) ({				    \
-	___type((src), a, ##__VA_ARGS__) __r;				    \
+	__u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))];		    \
 	BPF_CORE_READ_USER_INTO(&__r, (src), a, ##__VA_ARGS__);		    \
-	__r;								    \
+	*(___type((src), a, ##__VA_ARGS__) *)__r;			    \
 })
 
 /* Non-CO-RE variant of BPF_CORE_READ() */
 #define BPF_PROBE_READ(src, a, ...) ({					    \
-	___type((src), a, ##__VA_ARGS__) __r;				    \
+	__u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))];		    \
 	BPF_PROBE_READ_INTO(&__r, (src), a, ##__VA_ARGS__);		    \
-	__r;								    \
+	*(___type((src), a, ##__VA_ARGS__) *)__r;			    \
 })
 
 /*
@@ -552,9 +552,9 @@ extern void *bpf_rdonly_cast(const void *obj, __u32 btf_id) __ksym __weak;
  * not restricted to kernel types only.
  */
 #define BPF_PROBE_READ_USER(src, a, ...) ({				    \
-	___type((src), a, ##__VA_ARGS__) __r;				    \
+	__u8 __r[sizeof(___type((src), a, ##__VA_ARGS__))];		    \
 	BPF_PROBE_READ_USER_INTO(&__r, (src), a, ##__VA_ARGS__);	    \
-	__r;								    \
+	*(___type((src), a, ##__VA_ARGS__) *)__r;			    \
 })
 
 #endif