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bpf: fix bpf_probe_read_kernel prototype mismatch

PATCH Applied Arnd Bergmann 3 participants 3 messages Aug 1, 2023 – Aug 2, 2023

Subsystems (6)
Stable branch BPF Tracing BPF core +2 more subsystems
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Arnd Bergmann

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#0 lore raw

PATCH v5 1/1 bpf: fix bpf_probe_read_kernel prototype mismatch

From: Arnd Bergmann <[email protected]>

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.

As both versions of the function are fairly simple and only really differ in one line, just move them into a header file as an inline function that does not add any overhead for the bpf_trace.c callers and actually avoids a function call for the other one.

Cc [email protected]
Link https://lore.kernel.org/all/[email protected]/ open here
Signed-off-by Arnd Bergmann <[email protected]>

--
v5: rebase on top of 1f9a1ea821ff2 ("bpf: Support new sign-extension load insns")
v4: rewrite again to use a shared inline helper
v3: clarify changelog text further.
v2: rewrite completely to fix the mismatch.

---
 include/linux/bpf.h      | 12 ++++++++++++
 kernel/bpf/core.c        | 12 +++---------
 kernel/trace/bpf_trace.c | 11 -----------
 3 files changed, 15 insertions(+), 20 deletions(-)
include/linux/bpf.h +12 -0
index ceaa8c23287fc..abe75063630b8 100644
@@ -2661,6 +2661,18 @@ static inline void bpf_dynptr_set_rdonly(struct bpf_dynptr_kern *ptr)
 }
 #endif /* CONFIG_BPF_SYSCALL */
 
+static __always_inline int
+bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr)
+{
+	int ret = -EFAULT;
+
+	if (IS_ENABLED(CONFIG_BPF_EVENTS))
+		ret = copy_from_kernel_nofault(dst, unsafe_ptr, size);
+	if (unlikely(ret < 0))
+		memset(dst, 0, size);
+	return ret;
+}
+
 void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
 			  struct btf_mod_pair *used_btfs, u32 len);
kernel/bpf/core.c +3 -9
index 245402e2229da..52b2a01d74721 100644
@@ -1649,12 +1649,6 @@ 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)
-{
-	memset(dst, 0, size);
-	return -EFAULT;
-}
-
 /**
  *	___bpf_prog_run - run eBPF program on a given context
  *	@regs: is the array of MAX_BPF_EXT_REG eBPF pseudo-registers

@@ -2065,8 +2059,8 @@ 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;
 

@@ -2081,7 +2075,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 		DST = *(SIZE *)(unsigned long) (SRC + insn->off);	\
 		CONT;							\
 	LDX_PROBE_MEMSX_##SIZEOP:					\
-		bpf_probe_read_kernel(&DST, sizeof(SIZE),		\
+		bpf_probe_read_kernel_common(&DST, sizeof(SIZE),		\
 				      (const void *)(long) (SRC + insn->off));	\
 		DST = *((SIZE *)&DST);					\
 		CONT;
kernel/trace/bpf_trace.c +0 -11
index 79719979a6355..d6296d51a826a 100644
@@ -223,17 +223,6 @@ 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 ret;
-
-	ret = copy_from_kernel_nofault(dst, unsafe_ptr, size);
-	if (unlikely(ret < 0))
-		memset(dst, 0, size);
-	return ret;
-}
-
 BPF_CALL_3(bpf_probe_read_kernel, void *, dst, u32, size,
 	   const void *, unsafe_ptr)
 {
Signature · 1 line
2.39.2
reviewer Acked-by on v5 1/1
#1 lore raw
Quoted 22 lines · Arnd Bergmann

From: Arnd Bergmann <[email protected]>

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.

Quoted 8 lines

As both versions of the function are fairly simple and only really differ in one line, just move them into a header file as an inline function that does not add any overhead for the bpf_trace.c callers and actually avoids a function call for the other one.

Cc [email protected]
Link https://lore.kernel.org/all/[email protected]/ open here
Signed-off-by Arnd Bergmann <[email protected]>
Acked-by Yonghong Song <[email protected]>
CI Applied
#2 lore raw
In reply to Arnd Bergmann
This patch was applied to bpf/bpf-next.git (master) by Alexei Starovoitov <[email protected]>:

Hello:

This patch was applied to bpf/bpf-next.git (master) by Alexei Starovoitov <[email protected]>:

Quoted 10 lines · On Tue, 1 Aug 2023 13:13:58 +0200 you wrote:

From: Arnd Bergmann <[email protected]>

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:

[...]

Here is the summary with links:
- [v5] bpf: fix bpf_probe_read_kernel prototype mismatch

    https://git.kernel.org/bpf/bpf-next/c/6a5a148aaf14

You are awesome, thank you!

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