Re: [PATCH bpf-next v2 5/6] bpf: Introduce bpf_this_cpu_ptr()

On Thu, Sep 3, 2020 at 3:35 PM Hao Luo haoluo@google.com wrote:

Add bpf_this_cpu_ptr() to help access percpu var on this cpu. This helper always returns a valid pointer, therefore no need to check returned value for NULL. Also note that all programs run with preemption disabled, which means that the returned pointer is stable during all the execution of the program.

Signed-off-by: Hao Luo haoluo@google.com

looks good, few small things, but otherwise:

Acked-by: Andrii Nakryiko andriin@fb.com

include/linux/bpf.h | 1 + include/uapi/linux/bpf.h | 14 ++++++++++++++ kernel/bpf/verifier.c | 10 +++++++--- kernel/trace/bpf_trace.c | 14 ++++++++++++++ tools/include/uapi/linux/bpf.h | 14 ++++++++++++++ 5 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 6b2034f7665e..506fdd5d0463 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -307,6 +307,7 @@ enum bpf_return_type { RET_PTR_TO_ALLOC_MEM_OR_NULL, /* returns a pointer to dynamically allocated memory or NULL */ RET_PTR_TO_BTF_ID_OR_NULL, /* returns a pointer to a btf_id or NULL */ RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL, /* returns a pointer to a valid memory or a btf_id or NULL */

•   RET_PTR_TO_MEM_OR_BTF_ID,       /* returns a pointer to a valid memory or a btf_id */
  

};

/* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index d0ec94d5bdbf..e7ca91c697ed 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -3612,6 +3612,19 @@ union bpf_attr {

•         bpf_per_cpu_ptr() must check the returned value.
  

• Return
  

•         A generic pointer pointing to the kernel percpu variable on *cpu*.
  

• • void *bpf_this_cpu_ptr(const void *percpu_ptr)

• • Description
    

• •         Take a pointer to a percpu ksym, *percpu_ptr*, and return a
    

• •         pointer to the percpu kernel variable on this cpu. See the
    

• •         description of 'ksym' in **bpf_per_cpu_ptr**\ ().
    

• •         bpf_this_cpu_ptr() has the same semantic as this_cpu_ptr() in
    

• •         the kernel. Different from **bpf_per_cpu_ptr**\ (), it would
    

• •         never return NULL.
    

• • Return
    

• •         A generic pointer pointing to the kernel percpu variable on
    

what's "a generic pointer"? is it as opposed to sk_buff pointer or something?

• •         this cpu.
    

  */

#define __BPF_FUNC_MAPPER(FN) \ FN(unspec), \ @@ -3764,6 +3777,7 @@ union bpf_attr { FN(d_path), \ FN(copy_from_user), \ FN(bpf_per_cpu_ptr), \

•   FN(bpf_this_cpu_ptr),           \
    /* */
  
  

/* integer value in 'imm' field of BPF_CALL instruction selects which helper diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a702600ff581..e070d2abc405 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5016,8 +5016,10 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn regs[BPF_REG_0].type = PTR_TO_MEM_OR_NULL; regs[BPF_REG_0].id = ++env->id_gen; regs[BPF_REG_0].mem_size = meta.mem_size;

•   } else if (fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL) {
  

•   } else if (fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL ||
  

•              fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID) {
            const struct btf_type *t;
  

•           bool not_null = fn->ret_type == RET_PTR_TO_MEM_OR_BTF_ID;
  

nit: this is fine, but I'd inline it below

            mark_reg_known_zero(env, regs, BPF_REG_0);
            t = btf_type_skip_modifiers(btf_vmlinux, meta.ret_btf_id, NULL);

@@ -5034,10 +5036,12 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn tname, PTR_ERR(ret)); return -EINVAL; }

•                   regs[BPF_REG_0].type = PTR_TO_MEM_OR_NULL;
  

•                   regs[BPF_REG_0].type = not_null ?
  

•                           PTR_TO_MEM : PTR_TO_MEM_OR_NULL;
                    regs[BPF_REG_0].mem_size = tsize;
            } else {
  

•                   regs[BPF_REG_0].type = PTR_TO_BTF_ID_OR_NULL;
  

•                   regs[BPF_REG_0].type = not_null ?
  

•                           PTR_TO_BTF_ID : PTR_TO_BTF_ID_OR_NULL;
                    regs[BPF_REG_0].btf_id = meta.ret_btf_id;
            }
    } else if (fn->ret_type == RET_PTR_TO_BTF_ID_OR_NULL) {
  

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index d474c1530f87..466acf82a9c7 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -1160,6 +1160,18 @@ static const struct bpf_func_proto bpf_per_cpu_ptr_proto = { .arg2_type = ARG_ANYTHING, };

+BPF_CALL_1(bpf_this_cpu_ptr, const void *, percpu_ptr) +{

•   return (u64)this_cpu_ptr(percpu_ptr);
  

see previous comment, this might trigger unnecessary compilation warnings on 32-bit arches

+}

+static const struct bpf_func_proto bpf_this_cpu_ptr_proto = {

•   .func           = bpf_this_cpu_ptr,
  

•   .gpl_only       = false,
  

•   .ret_type       = RET_PTR_TO_MEM_OR_BTF_ID,
  

•   .arg1_type      = ARG_PTR_TO_PERCPU_BTF_ID,
  

+};

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