On 12/12/22 18:08, Yonghong Song wrote:
On 12/12/22 4:59 AM, Viktor Malik wrote:
When attaching fentry/fexit/fmod_ret/lsm to a function located in a
module without specifying the target program, the verifier tries to find
the address to attach to in kallsyms. This is always done by searching
the entire kallsyms, not respecting the module in which the function is
located.
This approach causes an incorrect attachment address to be computed if
the function to attach to is shadowed by a function of the same name
located earlier in kallsyms.
Since the attachment must contain the BTF of the program to attach to,
we may extract the module from it and search for the function address in
the module.
Signed-off-by: Viktor Malik <vmalik@xxxxxxxxxx>
---
kernel/bpf/verifier.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a5255a0dcbb6..d646c5263bc5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -24,6 +24,7 @@
#include <linux/bpf_lsm.h>
#include <linux/btf_ids.h>
#include <linux/poison.h>
+#include "../module/internal.h"
#include "disasm.h"
@@ -16478,6 +16479,7 @@ int bpf_check_attach_target(struct
bpf_verifier_log *log,
const char *tname;
struct btf *btf;
long addr = 0;
+ struct module *mod;
if (!btf_id) {
bpf_log(log, "Tracing programs must provide btf_id\n");
@@ -16645,7 +16647,19 @@ int bpf_check_attach_target(struct
bpf_verifier_log *log,
else
addr = (long) tgt_prog->aux->func[subprog]->bpf_func;
} else {
- addr = kallsyms_lookup_name(tname);
+ if (btf_is_module(btf)) {
+ preempt_disable();
+ mod = btf_try_get_module(btf);
+ if (mod) {
+ addr = find_kallsyms_symbol_value(mod, tname);
+ module_put(mod);
+ } else {
+ addr = 0;
+ }
+ preempt_enable();
What if module is unloaded right after preempt_enabled so 'addr' becomes
invalid? Is this a corner case we should consider?
IIUC, if 'addr' becomes invalid, the attachment will eventually fail.
So I'd say that there's no need to consider that case here, it's not
considered for kallsyms_lookup_name below (which may call
module_kallsyms_lookup_name) either.
+ } else {
+ addr = kallsyms_lookup_name(tname);
+ }
if (!addr) {
bpf_log(log,
"The address of function %s cannot be found\n",
