From: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
To: <sashiko-reviews@lists.linux.dev>
Cc: <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf-next 10/17] bpf: Recognize pointers to functions in read-only maps
Date: Tue, 22 Sep 2026 03:01:39 +0000 [thread overview]
Message-ID: <DLLIDTH3V5WR.3M19UWMDAHFMQ@gmail.com> (raw)
In-Reply-To: <20260922013100.6DF101F000FF@smtp.kernel.org>
On Tue, Sep 22, 2026 at 01:31 AM sashiko-bot@kernel.org <sashiko-bot@kernel.org> wrote:
>> + /*
>> + * The program that didn't load is not a user of the map. libbpf
>> + * loads the program again to get the log of the verifier.
>> + */
>> + cmpxchg(&map->user, (unsigned long)aux, 0);
>
> [Severity: Medium]
> Will this properly clear the map's user state if the BPF_MAP_USER_PATCHED
> flag was set before a failure?
>
> If jit_subprogs() sets the BPF_MAP_USER_PATCHED flag, map->user becomes
> me | BPF_MAP_USER_PATCHED. If a failure happens afterward, this cmpxchg()
> strictly expects map->user to equal aux, which will fail to match.
>
> This can leave the map permanently locked in the patched state, causing
> libbpf's subsequent reload attempt (to obtain the verifier log) to fail
> with -EBUSY.
No. Not a bug.
>> + } else if (verifier_bug_if(ptr->used, env, "function of map '%s' offset %u is removed",
>> + ptr->map->name, ptr->map_off)) {
>
> [Severity: High]
> Does eagerly setting ptr->used = true in check_func_ptr_read() lead to a
> verifier bug here if the function is never actually called?
>
> If a program simply reads the function pointer from the map but does not
> invoke callx, check_func_ptr_read() still flags it as used. Since the
> subprogram is never called, dead-code elimination can remove it.
>
> When jit_subprogs() later evaluates the deleted but "used" pointer, it
> incorrectly triggers a WARN_ONCE and returns -EFAULT, rejecting a valid
> program.
hmm. A prog can read the pointer and never call the
function, e.g. when the load is hoisted above a branch. Then the
function is dead, 'used' is set and jit_subprogs() hits
verifier_bug_if().
Will follow up with one line fix.
next prev parent reply other threads:[~2026-09-22 3:01 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-22 1:13 [PATCH bpf-next 00/17] bpf: Indirect calls of bpf subprogs (callx) Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 01/17] bpf: Fix infinite loop in check_max_stack_depth() Alexei Starovoitov
2026-09-22 2:01 ` bot+bpf-ci
2026-09-22 2:56 ` Alexei Starovoitov
2026-09-23 22:35 ` Eduard Zingerman
2026-09-22 1:13 ` [PATCH bpf-next 02/17] selftests/bpf: Test recursion through a global function and a callback Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 03/17] bpf: Don't fold loads from insn_array maps into constants Alexei Starovoitov
2026-09-23 22:39 ` Eduard Zingerman
2026-09-23 23:12 ` Alexei Starovoitov
2026-09-24 0:19 ` bot+bpf-ci
2026-09-22 1:13 ` [PATCH bpf-next 04/17] bpf: Prepare static analysis passes for callx instruction Alexei Starovoitov
2026-09-23 23:10 ` Eduard Zingerman
2026-09-23 23:51 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 05/17] bpf: Add callx instruction to call bpf subprogs indirectly Alexei Starovoitov
2026-09-24 0:09 ` Eduard Zingerman
2026-09-22 1:13 ` [PATCH bpf-next 06/17] bpf: Add callx calls to the call graph Alexei Starovoitov
2026-09-22 1:27 ` sashiko-bot
2026-09-22 2:54 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 07/17] bpf, x86: Add JIT support for callx Alexei Starovoitov
2026-09-22 1:27 ` sashiko-bot
2026-09-22 2:53 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 08/17] bpf, arm64: " Alexei Starovoitov
2026-09-22 15:05 ` Puranjay Mohan
2026-09-22 1:13 ` [PATCH bpf-next 09/17] bpf: Discover subprogs described by func_info Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 10/17] bpf: Recognize pointers to functions in read-only maps Alexei Starovoitov
2026-09-22 1:31 ` sashiko-bot
2026-09-22 3:01 ` Alexei Starovoitov [this message]
2026-09-24 0:46 ` bot+bpf-ci
2026-09-24 2:12 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 11/17] libbpf: Support pointers to static functions in data when linking Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 12/17] libbpf: Resolve pointers to functions in read-only data Alexei Starovoitov
2026-09-24 0:33 ` bot+bpf-ci
2026-09-24 2:13 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 13/17] libbpf: Treat .data.rel.ro as " Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 14/17] libbpf: Support pointers to functions in read-only data in light skeleton Alexei Starovoitov
2026-09-22 2:01 ` bot+bpf-ci
2026-09-22 2:55 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 15/17] selftests/bpf: Add tests for callx Alexei Starovoitov
2026-09-24 0:33 ` bot+bpf-ci
2026-09-24 2:13 ` Alexei Starovoitov
2026-09-22 1:13 ` [PATCH bpf-next 16/17] selftests/bpf: Add tests for callx through pointers in read-only data Alexei Starovoitov
2026-09-22 2:01 ` bot+bpf-ci
2026-09-24 0:33 ` bot+bpf-ci
2026-09-22 1:13 ` [PATCH bpf-next 17/17] bpf, docs: Document callx instruction Alexei Starovoitov
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