[PATCH 2/2] [v3] bpf: fix bpf_probe_read_kernel prototype mismatch

[Arnd Bergmann <arnd@xxxxxxxxxx>]

From: Arnd Bergmann <arnd@xxxxxxxx>

bpf_probe_read_kernel() has a __weak definition in core.c and another
definition with an incompatible prototype in kernel/trace/bpf_trace.c,
when CONFIG_BPF_EVENTS is enabled.

Since the two are incompatible, there cannot be a shared declaration in
a header file, but the lack of a prototype causes a W=1 warning:

kernel/bpf/core.c:1638:12: error: no previous prototype for 'bpf_probe_read_kernel' [-Werror=missing-prototypes]

On 32-bit architectures, the local prototype

u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)

passes arguments in other registers as the one in bpf_trace.c

BPF_CALL_3(bpf_probe_read_kernel, void *, dst, u32, size,
            const void *, unsafe_ptr)

which uses 64-bit arguments in pairs of registers.

Change the core.c file to only reference the inner
bpf_probe_read_kernel_common() helper and provide a prototype for that,
to ensure this is compatible with both definitions.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
--
v3: clarify changelog text further.
v2: rewrite completely to fix the mismatch.
---
 include/linux/bpf.h      | 2 ++
 kernel/bpf/core.c        | 9 ++++++---
 kernel/trace/bpf_trace.c | 3 +--
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f58895830adae..55826398acfba 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2619,6 +2619,8 @@ static inline void bpf_dynptr_set_rdonly(struct bpf_dynptr_kern *ptr)
 }
 #endif /* CONFIG_BPF_SYSCALL */
 
+int bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr);
+
 void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
 			  struct btf_mod_pair *used_btfs, u32 len);
 
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 0926714641eb5..565ef6950c7a8 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -35,6 +35,7 @@
 #include <linux/bpf_verifier.h>
 #include <linux/nodemask.h>
 #include <linux/nospec.h>
+#include <linux/bpf.h>
 #include <linux/bpf_mem_alloc.h>
 #include <linux/memcontrol.h>
 
@@ -1635,11 +1636,13 @@ bool bpf_opcode_in_insntable(u8 code)
 }
 
 #ifndef CONFIG_BPF_JIT_ALWAYS_ON
-u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr)
+#ifndef CONFIG_BPF_EVENTS
+int bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
 {
 	memset(dst, 0, size);
 	return -EFAULT;
 }
+#endif
 
 /**
  *	___bpf_prog_run - run eBPF program on a given context
@@ -1931,8 +1934,8 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 		DST = *(SIZE *)(unsigned long) (SRC + insn->off);	\
 		CONT;							\
 	LDX_PROBE_MEM_##SIZEOP:						\
-		bpf_probe_read_kernel(&DST, sizeof(SIZE),		\
-				      (const void *)(long) (SRC + insn->off));	\
+		bpf_probe_read_kernel_common(&DST, sizeof(SIZE),	\
+			      (const void *)(long) (SRC + insn->off));	\
 		DST = *((SIZE *)&DST);					\
 		CONT;
 
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 2bc41e6ac9fe0..290fdce2ce535 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -223,8 +223,7 @@ const struct bpf_func_proto bpf_probe_read_user_str_proto = {
 	.arg3_type	= ARG_ANYTHING,
 };
 
-static __always_inline int
-bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
+int bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
 {
 	int ret;
 
-- 
2.39.2