Date:   Mon, 4 Feb 2019 15:00:44 -0500
From:   Bruce Fields <bfields@fieldses.org>
To:     Chuck Lever <chuck.lever@oracle.com>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>, simo@redhat.com
Subject: Re: [PATCH RFC 09/10] SUNRPC: Remove xdr_buf_trim()
Message-ID: <20190204200044.GD1816@fieldses.org>
References: <20190201195538.11389.96106.stgit@manet.1015granger.net>
 <20190201195814.11389.4023.stgit@manet.1015granger.net>
 <20190204194615.GC1816@fieldses.org>
 <014F38E7-993C-4FD6-A8C6-9FE113A54A03@oracle.com>
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On Mon, Feb 04, 2019 at 02:49:11PM -0500, Chuck Lever wrote:
> 
> 
> > On Feb 4, 2019, at 2:46 PM, bfields@fieldses.org wrote:
> > 
> > On Fri, Feb 01, 2019 at 02:58:14PM -0500, Chuck Lever wrote:
> >> The key action of xdr_buf_trim() is that it shortens buf->len, the
> >> length of the xdr_buf' content. The other actions -- shortening the
> >> head, pages, and tail components -- are actually not necessary. In
> >> some cases, changing the size of those components corrupts the RPC
> >> message contained in the buffer.
> > 
> > That's really burying the lede.... Is there an actual user-visible bug
> > here?
> 
> I don't think so. This is more of the form:
> 
> a) the function does fundamentally the wrong thing, so
> 
> b) certain changes to this code path result is unexpected and incorrect
>    behavior
> 
> Thus typically only developers hacking on this code run into a problem.

OK, got it.  It'd help just to make it clear in the changelog that that
this is an accident waiting to happen rather than a current bug (as far
as we know).

--b.

> 
> 
> > --b.
> > 
> >> 
> >> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> >> ---
> >> include/linux/sunrpc/xdr.h          |    1 -
> >> net/sunrpc/auth_gss/gss_krb5_wrap.c |    8 ++++---
> >> net/sunrpc/auth_gss/svcauth_gss.c   |    2 +-
> >> net/sunrpc/xdr.c                    |   41 -----------------------------------
> >> 4 files changed, 6 insertions(+), 46 deletions(-)
> >> 
> >> diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
> >> index 69161cb..4ae398c 100644
> >> --- a/include/linux/sunrpc/xdr.h
> >> +++ b/include/linux/sunrpc/xdr.h
> >> @@ -183,7 +183,6 @@ static inline __be32 *xdr_encode_array(__be32 *p, const void *s, unsigned int le
> >> extern void xdr_shift_buf(struct xdr_buf *, size_t);
> >> extern void xdr_buf_from_iov(struct kvec *, struct xdr_buf *);
> >> extern int xdr_buf_subsegment(struct xdr_buf *, struct xdr_buf *, unsigned int, unsigned int);
> >> -extern void xdr_buf_trim(struct xdr_buf *, unsigned int);
> >> extern int xdr_buf_read_netobj(struct xdr_buf *, struct xdr_netobj *, unsigned int);
> >> extern int read_bytes_from_xdr_buf(struct xdr_buf *, unsigned int, void *, unsigned int);
> >> extern int write_bytes_to_xdr_buf(struct xdr_buf *, unsigned int, void *, unsigned int);
> >> diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c b/net/sunrpc/auth_gss/gss_krb5_wrap.c
> >> index 5cdde6c..14a0aff 100644
> >> --- a/net/sunrpc/auth_gss/gss_krb5_wrap.c
> >> +++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c
> >> @@ -570,14 +570,16 @@ static void rotate_left(u32 base, struct xdr_buf *buf, unsigned int shift)
> >> 	 */
> >> 	movelen = min_t(unsigned int, buf->head[0].iov_len, buf->len);
> >> 	movelen -= offset + GSS_KRB5_TOK_HDR_LEN + headskip;
> >> -	BUG_ON(offset + GSS_KRB5_TOK_HDR_LEN + headskip + movelen >
> >> -							buf->head[0].iov_len);
> >> +	if (offset + GSS_KRB5_TOK_HDR_LEN + headskip + movelen >
> >> +	    buf->head[0].iov_len)
> >> +		return GSS_S_FAILURE;
> >> 	memmove(ptr, ptr + GSS_KRB5_TOK_HDR_LEN + headskip, movelen);
> >> 	buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;
> >> 	buf->len -= GSS_KRB5_TOK_HDR_LEN + headskip;
> >> 
> >> 	/* Trim off the trailing "extra count" and checksum blob */
> >> -	xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);
> >> +	buf->len -= ec + GSS_KRB5_TOK_HDR_LEN + tailskip;
> >> +
> >> 	return GSS_S_COMPLETE;
> >> }
> >> 
> >> diff --git a/net/sunrpc/auth_gss/svcauth_gss.c b/net/sunrpc/auth_gss/svcauth_gss.c
> >> index 152790e..f1aabab 100644
> >> --- a/net/sunrpc/auth_gss/svcauth_gss.c
> >> +++ b/net/sunrpc/auth_gss/svcauth_gss.c
> >> @@ -896,7 +896,7 @@ u32 svcauth_gss_flavor(struct auth_domain *dom)
> >> 	if (svc_getnl(&buf->head[0]) != seq)
> >> 		goto out;
> >> 	/* trim off the mic and padding at the end before returning */
> >> -	xdr_buf_trim(buf, round_up_to_quad(mic.len) + 4);
> >> +	buf->len -= 4 + round_up_to_quad(mic.len);
> >> 	stat = 0;
> >> out:
> >> 	kfree(mic.data);
> >> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> >> index 5f0aa53..4bce619 100644
> >> --- a/net/sunrpc/xdr.c
> >> +++ b/net/sunrpc/xdr.c
> >> @@ -1139,47 +1139,6 @@ void xdr_enter_page(struct xdr_stream *xdr, unsigned int len)
> >> }
> >> EXPORT_SYMBOL_GPL(xdr_buf_subsegment);
> >> 
> >> -/**
> >> - * xdr_buf_trim - lop at most "len" bytes off the end of "buf"
> >> - * @buf: buf to be trimmed
> >> - * @len: number of bytes to reduce "buf" by
> >> - *
> >> - * Trim an xdr_buf by the given number of bytes by fixing up the lengths. Note
> >> - * that it's possible that we'll trim less than that amount if the xdr_buf is
> >> - * too small, or if (for instance) it's all in the head and the parser has
> >> - * already read too far into it.
> >> - */
> >> -void xdr_buf_trim(struct xdr_buf *buf, unsigned int len)
> >> -{
> >> -	size_t cur;
> >> -	unsigned int trim = len;
> >> -
> >> -	if (buf->tail[0].iov_len) {
> >> -		cur = min_t(size_t, buf->tail[0].iov_len, trim);
> >> -		buf->tail[0].iov_len -= cur;
> >> -		trim -= cur;
> >> -		if (!trim)
> >> -			goto fix_len;
> >> -	}
> >> -
> >> -	if (buf->page_len) {
> >> -		cur = min_t(unsigned int, buf->page_len, trim);
> >> -		buf->page_len -= cur;
> >> -		trim -= cur;
> >> -		if (!trim)
> >> -			goto fix_len;
> >> -	}
> >> -
> >> -	if (buf->head[0].iov_len) {
> >> -		cur = min_t(size_t, buf->head[0].iov_len, trim);
> >> -		buf->head[0].iov_len -= cur;
> >> -		trim -= cur;
> >> -	}
> >> -fix_len:
> >> -	buf->len -= (len - trim);
> >> -}
> >> -EXPORT_SYMBOL_GPL(xdr_buf_trim);
> >> -
> >> static void __read_bytes_from_xdr_buf(struct xdr_buf *subbuf, void *obj, unsigned int len)
> >> {
> >> 	unsigned int this_len;
> 
> --
> Chuck Lever
> 
>