From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9F11D1B0434 for ; Thu, 2 Jan 2025 13:21:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735824087; cv=none; b=rV4i6/nS1a0DFT+BadG31hNR9VjaJ/ZUydar7W/lXdwLyX3cZE8kW85mqEsyx0ODfXvjRvdQicEgXYmJgt3hxJAmP8fzuAqgHrqRt3OjoQaZu1Lrrj3wDM5R6d8poBwv+MroQlCi1Rgc/5YsF8Sx1OCw0cR9ZwaeNTn3SkSUMIU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735824087; c=relaxed/simple; bh=eFg24067G1/ru6VFPaq0wubXe1i6yDhwW+xiWepH098=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=Lc5PGeyMEmpOfbRRYG5+RatFbP2gA93s+RSO15O0TxBDdCHVt0X9HZAv7mimJXkUIUK6Y3N9qvrh/Za4RJDlhRDlJQcMPASh93/2ubiu5VBIJt6ve0XQ8AGCais2v5SBK0JFsFNlR+J1mLArb9nHnxjndAHzUsR40FGV4nmP3AA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=k+DMkTC4; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="k+DMkTC4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5EB9EC4CED7; Thu, 2 Jan 2025 13:21:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1735824087; bh=eFg24067G1/ru6VFPaq0wubXe1i6yDhwW+xiWepH098=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=k+DMkTC4oNhGsPESsIgVzEEkrNQ3p8ZjpxckkNksr5BdSK7smr4AeD2+wQfC8pXdO JwZeferuHJxTK2PRv9ndF9QTE/wtnPeXer1U4v1dOtPVFHa9tQaf/USml32edydH/d MgkzT4zJPMe0dlzR+4dJzS9BLF7PzOJfxXAlR8vC+2xdpcnSzDg3FqmpLJkgr89Pla UKyvIh8VZj4QkDUDvm9dbxztXWFgNWCjtD5WnKFw9G/3BVepU0EFhzf4Cv6r3VLbPc v7mBJBd6EYvj8pHiUcafSgRPJwEthO5YfxpRz541hQnPJGUDV4YFADFy59NeEtSJmy 1Ud38in+A6S2Q== Message-ID: <6a262addf1c7996a93e4e6d97493056a919c4f03.camel@kernel.org> Subject: Re: [PATCH v4 9/9] SUNRPC: Document validity guarantees of the pointer returned by reserve_space From: Jeff Layton To: cel@kernel.org, Neil Brown , Olga Kornievskaia , Dai Ngo , Tom Talpey Cc: linux-nfs@vger.kernel.org, Rick Macklem , j.david.lists@gmail.com, Chuck Lever Date: Thu, 02 Jan 2025 08:21:25 -0500 In-Reply-To: <20241231002901.12725-10-cel@kernel.org> References: <20241231002901.12725-1-cel@kernel.org> <20241231002901.12725-10-cel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Mon, 2024-12-30 at 19:29 -0500, cel@kernel.org wrote: > From: Chuck Lever >=20 > A subtlety of this API is that if the @nbytes region traverses a > page boundary, the next __xdr_commit_encode will shift the data item > in the XDR encode buffer. This makes the returned pointer point to > something else, leading to unexpected behavior. >=20 > There are a few cases where the caller saves the returned pointer > and then later uses it to insert a computed value into an earlier > part of the stream. This can be safe only if either: >=20 > - the data item is guaranteed to be in the XDR buffer's head, and > thus is not ever going to be near a page boundary, or > - the data item is no larger than 4 octets, since XDR alignment > rules require all data items to start on 4-octet boundaries >=20 > But that safety is only an artifact of the current implementation. > It would be less brittle if these "safe" uses were eventually > replaced. >=20 > Signed-off-by: Chuck Lever > --- > net/sunrpc/xdr.c | 6 ++++++ > 1 file changed, 6 insertions(+) >=20 > diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c > index 62e07c330a66..4e003cb516fe 100644 > --- a/net/sunrpc/xdr.c > +++ b/net/sunrpc/xdr.c > @@ -1097,6 +1097,12 @@ static noinline __be32 *xdr_get_next_encode_buffer= (struct xdr_stream *xdr, > * Checks that we have enough buffer space to encode 'nbytes' more > * bytes of data. If so, update the total xdr_buf length, and > * adjust the length of the current kvec. > + * > + * The returned pointer is valid only until the next call to > + * xdr_reserve_space() or xdr_commit_encode() on @xdr. The current > + * implementation of this API guarantees that space reserved for a > + * four-byte data item remains valid until @xdr is destroyed, but > + * that might not always be true in the future. > */ > __be32 * xdr_reserve_space(struct xdr_stream *xdr, size_t nbytes) > { I agree with Neil that this API could do with less footguns, but this does seem to be an improvement. Reviewed-by: Jeff Layton