Merge branch 'tracing-fix-kprobe-attachment-when-module-shadows-vmlinux-symbol'

Andrey Grodzovsky says:

====================
tracing: Fix kprobe attachment when module shadows vmlinux symbol

When a kernel module exports a symbol with the same name as an existing
vmlinux symbol, kprobe attachment fails with -EADDRNOTAVAIL because
number_of_same_symbols() counts matches across both vmlinux and all
loaded modules, returning a count greater than 1.

This series takes a different approach from v1-v4, which implemented a
libbpf-side fallback parsing /proc/kallsyms and retrying with the
absolute address. That approach was rejected (Andrii Nakryiko, Ihor
Solodrai) because ambiguous symbol resolution does not belong in libbpf.

Following Ihor's suggestion, this series fixes the root cause in the
kernel: when an unqualified symbol name is given and the symbol is found
in vmlinux, prefer the vmlinux symbol and do not scan loaded modules.
This makes the skeleton auto-attach path work transparently with no
libbpf changes needed.

Patch 1: Kernel fix - return vmlinux-only count from
         number_of_same_symbols() when the symbol is found in vmlinux,
         preventing module shadows from causing -EADDRNOTAVAIL.
Patch 2: Selftests using bpf_fentry_shadow_test which exists in both
         vmlinux and bpf_testmod - tests unqualified (vmlinux) and
         MOD:SYM (module) attachment across all four attach modes, plus
         kprobe_multi with the duplicate symbol.

Changes since v6 [1]:
  - Fix comment style: use /* on its own line instead of networking-style
    /* text on opener line (Alexei Starovoitov).

[1] https://lore.kernel.org/bpf/20260407165145.1651061-1-andrey.grodzovsky@crowdstrike.com/
====================

Link: https://patch.msgid.link/20260407203912.1787502-1-andrey.grodzovsky@crowdstrike.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index a5dbb72528e0..058724c41c46 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -765,6 +765,14 @@ static unsigned int number_of_same_symbols(const char *mod, const char *func_nam
 	if (!mod)
 		kallsyms_on_each_match_symbol(count_symbols, func_name, &ctx.count);
 
+	/*
+	 * If the symbol is found in vmlinux, use vmlinux resolution only.
+	 * This prevents module symbols from shadowing vmlinux symbols
+	 * and causing -EADDRNOTAVAIL for unqualified kprobe targets.
+	 */
+	if (!mod && ctx.count > 0)
+		return ctx.count;
+
 	module_kallsyms_on_each_symbol(mod, count_mod_symbols, &ctx);
 
 	return ctx.count;