[PATCH bpf-next] bpftool: Resolve libcrypto link flags via pkg-config

[hadrien Patte <hadrien.patte@protonmail.com>]

From: Hadrien Patte <hadrien.patte@protonmail.com>

When building bpftool with EXTRA_LDFLAGS=-static on Ubuntu 26.04
(OpenSSL 3.5.5), the link fails because libcrypto.a references
libjitterentropy symbols, which are not on the link line:

  LINK     bpftool
  /usr/bin/ld: libcrypto.a(libdefault-lib-seed_src_jitter.o):
    in function `ossl_prov_acquire_entropy_from_jitter':
    (.text+0x11c): undefined reference to `jent_entropy_collector_alloc'
    (.text+0x130): undefined reference to `jent_read_entropy'
    (.text+0x13c): undefined reference to `jent_entropy_collector_free'
  /usr/bin/ld: libcrypto.a(libdefault-lib-seed_src_jitter.o):
    in function `jitter_instantiate':
    (.text+0x61c): undefined reference to `jent_entropy_init_ex'
  collect2: error: ld returned 1 exit status

The set of transitive dependencies pulled in by libcrypto.a varies
across distributions and OpenSSL builds (cf. commit 08a749184322
("bpftool: Fix dependencies for static build"), which addressed a
similar libz ordering issue), so hard-coding them in the Makefile is
fragile.

Resolve libcrypto's link flags via pkg-config, which knows about
these transitive deps. Pass --static when the user requested a static
build via EXTRA_LDFLAGS=-static so pkg-config emits the static link
line (e.g. "-lcrypto -l:libjitterentropy.a -lz -lzstd -ldl -pthread"
on Ubuntu 26.04). Fall back to -lcrypto if pkg-config or
libcrypto.pc is unavailable, preserving the previous behavior on
minimal build environments.

Signed-off-by: Hadrien Patte <hadrien.patte@protonmail.com>
---
 tools/bpf/bpftool/Makefile | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 0febf60e1b64..d9ab860a6cce 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -103,7 +103,17 @@ SKIP_LLVM ?=
 SKIP_LIBBFD ?=
 SKIP_CRYPTO ?=
 ifneq ($(SKIP_CRYPTO),1)
-  CRYPTO_LIBS := -lcrypto
+  # Resolve libcrypto link flags via pkg-config so transitive dependencies
+  # (e.g. libjitterentropy on distros where libcrypto.a references it) are
+  # included for static builds. Pass --static when the user requested a
+  # static build via EXTRA_LDFLAGS=-static. Fall back to -lcrypto if
+  # pkg-config or libcrypto.pc is unavailable.
+  ifneq ($(findstring -static,$(EXTRA_LDFLAGS)),)
+    CRYPTO_LIBS := $(shell pkg-config --libs --static libcrypto 2>/dev/null)
+  else
+    CRYPTO_LIBS := $(shell pkg-config --libs libcrypto 2>/dev/null)
+  endif
+  CRYPTO_LIBS := $(if $(CRYPTO_LIBS),$(CRYPTO_LIBS),-lcrypto)
 endif
 
 FEATURE_TESTS := clang-bpf-co-re
-- 
2.54.0