From nobody Wed Sep 02 14:30:30 2026 X-Original-To: dev-commits-src-all@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4hZlWc0xcKz6q8dQ for ; Wed, 02 Sep 2026 14:30:36 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from mxrelay.nyi.freebsd.org (mxrelay.nyi.freebsd.org [IPv6:2610:1c1:1:606c::19:3]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "mxrelay.nyi.freebsd.org", Issuer "YR2" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id 4hZlWb6lJBz3T66 for ; Wed, 02 Sep 2026 14:30:35 +0000 (UTC) (envelope-from git@FreeBSD.org) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1788359436; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=f3ESDQHY4S9Uyxwhsl45ZhrNOaFONRQONbkdJNhcn70=; b=DZO/46T4fAeGGW0S4YfSvs1oiXO5Q8GHuCQvRgWZilRGiYzzbAZDaitqtHkxbSocHVun0a A1o9Y7qj3wbO4UBLYRS+0O4H1QhcIpybwwRU1leDByENyc7qi2GAoBBn5qThP6RIxYdX8w T4bnSUHg3XfJ4us0PZNXFmJncgLDYcnreJSD8xxRCObIOmoHoV/rkRzYVr2pHu1wr0TxJ4 0WUeqon7RpelO5dxRVi9wT+xP/VckgHYV7/oaFI8qb20hk6KZMEapz5di7BhOsiZ7nPnjp s18scnai/WYsqtiyTxzIUzUr8ZAwYHs8rxIDhdr5e5yGaFNSuWbSSlkf/ChfUQ== ARC-Seal: i=1; s=dkim; d=freebsd.org; t=1788359436; a=rsa-sha256; cv=none; b=mIrmEm3cRzjp8QS+MwxsFgZ7mo0U3sw7YBNGR0qtiRzhf6vUakvUhF4uUEAgpybynFOUZS WCWl+Y+i5vXEH8sZv49djNe6NnZRJH/oDIUA5U1dx/X+fQV5MtZ/cnKHKfXLKfTCcP1Ol3 EIggJFjtnqqPLF16R3mW+Pm9cCbowJgCeFJQoWnGvQaqDAZfQiVH82e623EDigpjp10DCP 0uZ31dXlWwGruNKlOdZz3Ti1gb35BvtuSdISpN0vacMtFXYORDfW4prdLEgbLstF6rMRzC cs0Oc73MuHZchVCXfi4lfLF0wacCcMGFoU9B4GJg4DBUPqmjACnuEZW6VgvaEQ== ARC-Authentication-Results: i=1; mx1.freebsd.org; none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1788359436; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=f3ESDQHY4S9Uyxwhsl45ZhrNOaFONRQONbkdJNhcn70=; b=GE7zgnOVV/280NQeE8gTcyUk6anQS0xJeL8MDn2WQihOX6R7ieXoQHiiQnfhKQrKQ6qx/i 1eieXh9TrJW5AQ5fuRbaszBzyBOs2j36O3knnfvFcGwK5wt7dXDz3cVIcwkP2/qohPXY+E fyFi8ALWuzu1r/neHtA6yMLFXxxeeQpSV8JlB5yj3fl00yKlTEe+geHnTdId5OAO0H0FYN RHePcCQD59YvWPONpCxRuWhaCvknxR33fUy1dbBuBYC5uW1EsLxk4HxAYWWu3/gwO+an6X 7QkkypUDpZda/OXk0+MMstvzLuWj52+QSZhHtA2HXusI8UIcrYCWwQYeVXyr/Q== Received: from gitrepo.freebsd.org (gitrepo.freebsd.org [IPv6:2610:1c1:1:6068::e6a:5]) by mxrelay.nyi.freebsd.org (Postfix) with ESMTP id 4hZlWb5SjYznZp for ; Wed, 02 Sep 2026 14:30:35 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from git (uid 1279) (envelope-from git@FreeBSD.org) id 22e29 by gitrepo.freebsd.org (DragonFly Mail Agent v0.13+ on gitrepo.freebsd.org); Wed, 02 Sep 2026 14:30:30 +0000 To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Cc: Nick Banks From: Mitchell Horne Subject: git: 1db0c9fa7a32 - main - hwpmc(4): stop overwalking past _start on amd64 user callchains List-Id: Commit messages for all branches of the src repository List-Archive: https://lists.freebsd.org/archives/dev-commits-src-all List-Help: List-Post: List-Subscribe: List-Unsubscribe: X-BeenThere: dev-commits-src-all@freebsd.org Sender: owner-dev-commits-src-all@FreeBSD.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Git-Committer: mhorne X-Git-Repository: src X-Git-Refname: refs/heads/main X-Git-Reftype: branch X-Git-Commit: 1db0c9fa7a32255c452c2536d657e63b55e30e03 Auto-Submitted: auto-generated Date: Wed, 02 Sep 2026 14:30:30 +0000 Message-Id: <6a983306.22e29.c6d9ec6@gitrepo.freebsd.org> The branch main has been updated by mhorne: URL: https://cgit.FreeBSD.org/src/commit/?id=1db0c9fa7a32255c452c2536d657e63b55e30e03 commit 1db0c9fa7a32255c452c2536d657e63b55e30e03 Author: Nick Banks AuthorDate: 2026-09-02 14:28:54 +0000 Commit: Mitchell Horne CommitDate: 2026-09-02 14:29:08 +0000 hwpmc(4): stop overwalking past _start on amd64 user callchains pmc_save_user_callchain() emits the pc it just loaded before checking whether fp is the ABI's zero frame-chain terminator. At the bottom of a well-formed chain under _start, fp comes back 0 as expected, but the paired pc is stale rtld data left on the stack -- a legal userspace VA that still passes PMC_IN_USERSPACE(), so it gets emitted as a bogus extra frame. This shows up in flame graphs as a spurious hex-valued root frame below _start. Check fp == 0 alongside the existing checks before emitting, matching how arm/arm64/powerpc already load the next fp before their check. Measured via 1kHz hwpmc sampling on an OCA: stacks with any unresolved hex frame drop from 23.9% to 1.3%, and stacks with hex at the root drop from 5.5% to 0.3%. Reviewed by: mhorne, Ali Mashtizadeh , gallatin MFC after: 3 days Sponsored by: Netflix Differential Revision: https://reviews.freebsd.org/D59229 --- sys/dev/hwpmc/hwpmc_x86.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/sys/dev/hwpmc/hwpmc_x86.c b/sys/dev/hwpmc/hwpmc_x86.c index 2903c25ef5c9..c32242a2bbb9 100644 --- a/sys/dev/hwpmc/hwpmc_x86.c +++ b/sys/dev/hwpmc/hwpmc_x86.c @@ -107,7 +107,8 @@ pmc_save_user_callchain(uintptr_t *cc, int nframes, struct trapframe *tf) return (n); for (; n < nframes;) { - if (pc == 0 || !PMC_IN_USERSPACE(pc)) + /* fp == 0 is the ABI frame-chain sentinel below _start. */ + if (pc == 0 || !PMC_IN_USERSPACE(pc) || fp == 0) break; *cc++ = pc; n++;