On 7/29/21 9:49 PM, Andrii Nakryiko wrote:

On Thu, Jul 29, 2021 at 11:17 AM Yonghong Song [off-list ref] wrote:

quoted

On 7/26/21 9:12 AM, Andrii Nakryiko wrote:

quoted

Add new BPF helper, bpf_get_user_ctx(), which can be used by BPF programs to
get access to the user_ctx value, specified during BPF program attachment (BPF
link creation) time.

Currently all perf_event-backed BPF program types support bpf_get_user_ctx()
helper. Follow-up patches will add support for fentry/fexit programs as well.

While at it, mark bpf_tracing_func_proto() as static to make it obvious that
it's only used from within the kernel/trace/bpf_trace.c.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
   include/linux/bpf.h            |  3 ---
   include/uapi/linux/bpf.h       | 16 ++++++++++++++++
   kernel/trace/bpf_trace.c       | 35 +++++++++++++++++++++++++++++++++-
   tools/include/uapi/linux/bpf.h | 16 ++++++++++++++++
   4 files changed, 66 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 74b35faf0b73..94ebedc1e13a 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -2110,9 +2110,6 @@ extern const struct bpf_func_proto bpf_btf_find_by_name_kind_proto;
   extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
   extern const struct bpf_func_proto bpf_sk_getsockopt_proto;

-const struct bpf_func_proto *bpf_tracing_func_proto(
-     enum bpf_func_id func_id, const struct bpf_prog *prog);
-
   const struct bpf_func_proto *tracing_prog_func_proto(
     enum bpf_func_id func_id, const struct bpf_prog *prog);

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index bc1fd54a8f58..96afeced3467 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -4856,6 +4856,21 @@ union bpf_attr {
    *          Get address of the traced function (for tracing and kprobe programs).
    *  Return
    *          Address of the traced function.
+ *
+ * u64 bpf_get_user_ctx(void *ctx)
+ *   Description
+ *           Get user_ctx value provided (optionally) during the program
+ *           attachment. It might be different for each individual
+ *           attachment, even if BPF program itself is the same.
+ *           Expects BPF program context *ctx* as a first argument.
+ *
+ *           Supported for the following program types:
+ *                   - kprobe/uprobe;
+ *                   - tracepoint;
+ *                   - perf_event.

I think it is possible in the future we may need to support more
program types with user_ctx, not just u64 but more than 64bit value.
Should we may make this helper extensible like
      long bpf_get_user_ctx(void *ctx, void *user_ctx, u32 user_ctx_len)

The return value will 0 to be good and a negative indicating an error.
What do you think?

I explicitly wanted to keep this user_ctx/bpf_cookie to a small fixed
size. __u64 is perfect because it's small enough to not require
dynamic memory allocation, but big enough to store any kind of index
into an array *or* user-space pointer. So if user needs more storage
than 8 bytes, they will be able to have a bigger array where
user_ctx/bpf_cookie is just an integer index or some sort of key into
hashmap, whichever is more convenient.

Okay. returning an index to a map is a good idea. This way, indeed a u64 
return value is enough.

So I'd like to keep it lean and simple. It is already powerful enough
to support any scenario, IMO.

quoted

quoted

+ *   Return
+ *           Value specified by user at BPF link creation/attachment time
+ *           or 0, if it was not specified.
    */
   #define __BPF_FUNC_MAPPER(FN)               \
       FN(unspec),                     \

@@ -5032,6 +5047,7 @@ union bpf_attr {
       FN(timer_start),                \
       FN(timer_cancel),               \
       FN(get_func_ip),                \
+     FN(get_user_ctx),               \
       /* */

   /* integer value in 'imm' field of BPF_CALL instruction selects which helper

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index c9cf6a0d0fb3..b14978b3f6fb 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -975,7 +975,34 @@ static const struct bpf_func_proto bpf_get_func_ip_proto_kprobe = {
       .arg1_type      = ARG_PTR_TO_CTX,
   };

-const struct bpf_func_proto *
+BPF_CALL_1(bpf_get_user_ctx_trace, void *, ctx)
+{
+     struct bpf_trace_run_ctx *run_ctx;
+
+     run_ctx = container_of(current->bpf_ctx, struct bpf_trace_run_ctx, run_ctx);
+     return run_ctx->user_ctx;
+}
+
+static const struct bpf_func_proto bpf_get_user_ctx_proto_trace = {
+     .func           = bpf_get_user_ctx_trace,
+     .gpl_only       = false,
+     .ret_type       = RET_INTEGER,
+     .arg1_type      = ARG_PTR_TO_CTX,
+};
+

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