netdev - [RFC PATCH bpf-next 4/7] bpftool: stop emitting static variables in BPF skeleton

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Date:   Thu, 6 May 2021 22:41:16 -0700
From:   Andrii Nakryiko <andrii@...nel.org>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii@...nel.org>, <kernel-team@...com>
Subject: [RFC PATCH bpf-next 4/7] bpftool: stop emitting static variables in BPF skeleton

As discussed in [0], stop emitting static variables in BPF skeletons to avoid
issues with name-conflicting static variables across multiple
statically-linked BPF object files.

Users using static variables to pass data between BPF programs and user-space
should do a trivial one-time switch according to the following simple rules:
  - read-only `static volatile const` variables should be converted to
    `volatile const`;
  - read/write `static volatile` variables should just drop `static volatile`
    modifiers to become global variables/symbols. To better handle older Clang
    versions, such newly converted global variables should be explicitly
    initialized with a specific value or `= 0`/`= {}`, whichever is
    appropriate.

  [0] https://lore.kernel.org/bpf/CAEf4BzZo7_r-hsNvJt3w3kyrmmBJj7ghGY8+k4nvKF0KLjma=w@mail.gmail.com/T/#m664d4b0d6b31ac8b2669360e0fc2d6962e9f5ec1

Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
---
 tools/bpf/bpftool/gen.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 06fee4a2910a..27dceaf66ecb 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -131,6 +131,10 @@ static int codegen_datasec_def(struct bpf_object *obj,
 		int need_off = sec_var->offset, align_off, align;
 		__u32 var_type_id = var->type;
 
+		/* static variables are not exposed through BPF skeleton */
+		if (btf_var(var)->linkage == BTF_VAR_STATIC)
+			continue;
+
 		if (off > need_off) {
 			p_err("Something is wrong for %s's variable #%d: need offset %d, already at %d.\n",
 			      sec_name, i, need_off, off);
-- 
2.30.2