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

Arnd Bergmann
2023-06-02T13:52:03Z

From: Arnd Bergmann <arnd@arndb.de>

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@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
--
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