Re: [PATCH] [v4] bpf: fix bpf_probe_read_kernel prototype mismatch

On 7/25/23 1:41 PM, Arnd Bergmann wrote:

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.

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: stable@vger.kernel.org Link: https://lore.kernel.org/all/ac25cb0f-b804-1649-3afb-1dc6138c2716@iogearbox.n... Signed-off-by: Arnd Bergmann arnd@arndb.de

LGTM but there are some additional changes in kernel so you need to rebase on top of master branch with the following additional change:

@@ -2082,7 +2076,7 @@ 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;

Thanks!

-- 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 | 10 ++-------- kernel/trace/bpf_trace.c | 11 ----------- 3 files changed, 14 insertions(+), 19 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h index ceaa8c23287fc..abe75063630b8 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -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);

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index dd70c58c9d3a3..9cdf53bfb8bd3 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -1634,12 +1634,6 @@ 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

@@ -1930,8 +1924,8 @@ 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 c92eb8c6ff08d..83bde2475ae54 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -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) {