Re: [PATCH v2 3/3] NFSD: Clean up symlink argument XDR decoders

[Bruce Fields <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>]

On Tue, Jan 23, 2018 at 02:30:57PM -0800, Chuck Lever wrote:
> 
> 
> > On Jan 23, 2018, at 12:52 PM, Bruce Fields <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org> wrote:
> > 
> > On Mon, Jan 22, 2018 at 05:09:35PM -0800, Chuck Lever wrote:
> >> 
> >> 
> >>> On Jan 22, 2018, at 2:00 PM, J. Bruce Fields <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org> wrote:
> >>> 
> >>> On Wed, Jan 03, 2018 at 03:42:35PM -0500, Chuck Lever wrote:
> >>>> Move common code in NFSD's symlink arg decoders into a helper. The
> >>>> immediate benefits include:
> >>>> 
> >>>> - one fewer data copies on transports that support DDP
> >>>> - no memory allocation in the NFSv4 XDR decoder
> >>>> - consistent error checking across all versions
> >>>> - reduction of code duplication
> >>>> - support for both legal forms of SYMLINK requests on RDMA
> >>>>  transports for all versions of NFS (in particular, NFSv2, for
> >>>>  completeness)
> >>>> 
> >>>> In the long term, this helper is an appropriate spot to perform a
> >>>> per-transport call-out to fill the pathname argument using, say,
> >>>> RDMA Reads.
> >>>> 
> >>>> Filling the pathname in the proc function also means that eventually
> >>>> the incoming filehandle can be interpreted so that filesystem-
> >>>> specific memory can be allocated as a sink for the pathname
> >>>> argument, rather than using anonymous pages.
> >>>> 
> >>>> Wondering why the current code punts a zero-length SYMLINK. Is it
> >>>> illegal to create a zero-length SYMLINK on Linux?
> >>> 
> >>> SYMLINK(2) says
> >>> 
> >>> 	ENOENT A directory component in linkpath does not exist or is a
> >>> 	dangling symbolic link, or target or linkpath is an empty
> >>> 	string.
> >>> 
> >>> That doesn't explain the INVAL, or why this is the right place to be
> >>> checking it.
> >> 
> >> RFC 1813:
> >> 
> >> NFS3ERR_IO
> >>      NFS3ERR_ACCES
> >>      NFS3ERR_EXIST
> >>      NFS3ERR_NOTDIR
> >>      NFS3ERR_NOSPC
> >>      NFS3ERR_ROFS
> >>      NFS3ERR_NAMETOOLONG
> >>      NFS3ERR_DQUOT
> >>      NFS3ERR_STALE
> >>      NFS3ERR_BADHANDLE
> >>      NFS3ERR_NOTSUPP
> >>      NFS3ERR_SERVERFAULT
> >> 
> >> Interestingly, neither INVAL nor NOENT are valid
> >> status codes for NFSv3 SYMLINK. NFS3ERR_NOTSUPP
> >> might be closest, I suppose.
> >> 
> >> RFC 5661 says explicitly:
> >> 
> >> If the objname has a length of zero, or if objname does not obey the
> >> UTF-8 definition, the error NFS4ERR_INVAL will be returned.
> >> 
> >> And lists these as valid status codes for CREATE(NF4LNK):
> >> 
> >> | NFS4ERR_ACCESS, NFS4ERR_ATTRNOTSUPP,       |
> >> | NFS4ERR_BADCHAR, NFS4ERR_BADNAME,          |
> >> | NFS4ERR_BADOWNER, NFS4ERR_BADTYPE,         |
> >> | NFS4ERR_BADXDR, NFS4ERR_DEADSESSION,       |
> >> | NFS4ERR_DELAY, NFS4ERR_DQUOT,              |
> >> | NFS4ERR_EXIST, NFS4ERR_FHEXPIRED,          |
> >> | NFS4ERR_INVAL, NFS4ERR_IO, NFS4ERR_MLINK,  |
> >> | NFS4ERR_MOVED, NFS4ERR_NAMETOOLONG,        |
> >> | NFS4ERR_NOFILEHANDLE, NFS4ERR_NOSPC,       |
> >> | NFS4ERR_NOTDIR, NFS4ERR_OP_NOT_IN_SESSION, |
> >> | NFS4ERR_PERM, NFS4ERR_REP_TOO_BIG,         |
> >> | NFS4ERR_REP_TOO_BIG_TO_CACHE,              |
> >> | NFS4ERR_REQ_TOO_BIG,                       |
> >> | NFS4ERR_RETRY_UNCACHED_REP, NFS4ERR_ROFS,  |
> >> | NFS4ERR_SERVERFAULT, NFS4ERR_STALE,        |
> >> | NFS4ERR_TOO_MANY_OPS,                      |
> >> | NFS4ERR_UNSAFE_COMPOUND                    |
> >> 
> >> 
> >>> I'm a little nervous about the NULL termination in
> >>> svc_fill_symlink_pathname; how do we know it's safe to write a zero
> >>> there?  I haven't checked it carefully yet.
> >> 
> >> svc_fill_symlink_pathname grabs a whole fresh page
> >> from @rqstp. It is safe to write bytes anywhere in
> >> that page.
> > 
> > How do we know it's safe to grab another page?
> 
> It's safe in the v3 case because that's what the v3
> decoder currently does. I think you are concerned
> about the NFSv4 COMPOUND case on TCP, for arbitrarily
> large compounds? Given that we have 257-ish pages
> in the rqstp->rq_pages array (ie, that number is
> bounded) I'm not sure how we can ever be sure there
> are enough pages for all combinations of NFS
> operations.

Right, I think it just depends on some maximum lengths being checked
earlier.  So my worry is that this my violate some assumptions about how
many pages we need to handle a request of a certain length.

I'll go remind myself how this works.  We need some documentation here
at least.  So I may end up postponing these after all....

> > (Could we run out of
> > pages?  I'd probably know the answer if I'd rewritten this code
> > recently, but I haven't and the details are swapped out....)
> > 
> > Also I don't think that's true in the first->iov_base == 0 case.
> 
> This case occurs when an RDMA transport has filled
> in the first page of arg->pages with the symlink
> contents. It's also a whole fresh page that was
> obtained from rqstp->rq_pages.

OK, I wasn't following that case.

--b.

> 
> Here, are you concerned about the client sending a
> COMPOUND over RPC/RDMA that has more than one SYMLINK
> operation? Our server currently supports only one Read
> chunk per RPC.
> 
> 
> > --b.
> > 
> >> 
> >> 
> >>> --g.
> >>> 
> >>>> 
> >>>> Signed-off-by: Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> >>>> ---
> >>>> fs/nfsd/nfs3proc.c         |   10 +++++++
> >>>> fs/nfsd/nfs3xdr.c          |   51 ++++++++-------------------------
> >>>> fs/nfsd/nfs4proc.c         |    7 +++++
> >>>> fs/nfsd/nfs4xdr.c          |   10 +++++--
> >>>> fs/nfsd/nfsproc.c          |   14 +++++----
> >>>> fs/nfsd/nfsxdr.c           |   49 +++++++++++++++++++-------------
> >>>> fs/nfsd/xdr.h              |    1 +
> >>>> fs/nfsd/xdr3.h             |    1 +
> >>>> fs/nfsd/xdr4.h             |    2 +
> >>>> include/linux/sunrpc/svc.h |    2 +
> >>>> net/sunrpc/svc.c           |   67 ++++++++++++++++++++++++++++++++++++++++++++
> >>>> 11 files changed, 146 insertions(+), 68 deletions(-)
> >>>> 
> >>>> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
> >>>> index 2dd95eb..6259a4b 100644
> >>>> --- a/fs/nfsd/nfs3proc.c
> >>>> +++ b/fs/nfsd/nfs3proc.c
> >>>> @@ -283,6 +283,16 @@
> >>>> 	struct nfsd3_diropres *resp = rqstp->rq_resp;
> >>>> 	__be32	nfserr;
> >>>> 
> >>>> +	if (argp->tlen == 0)
> >>>> +		RETURN_STATUS(nfserr_inval);
> >>>> +	if (argp->tlen > NFS3_MAXPATHLEN)
> >>>> +		RETURN_STATUS(nfserr_nametoolong);
> >>>> +
> >>>> +	argp->tname = svc_fill_symlink_pathname(rqstp, &argp->first,
> >>>> +						argp->tlen);
> >>>> +	if (IS_ERR(argp->tname))
> >>>> +		RETURN_STATUS(nfserrno(PTR_ERR(argp->tname)));
> >>>> +
> >>>> 	dprintk("nfsd: SYMLINK(3)  %s %.*s -> %.*s\n",
> >>>> 				SVCFH_fmt(&argp->ffh),
> >>>> 				argp->flen, argp->fname,
> >>>> diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
> >>>> index 240cdb0e..78b555b 100644
> >>>> --- a/fs/nfsd/nfs3xdr.c
> >>>> +++ b/fs/nfsd/nfs3xdr.c
> >>>> @@ -452,51 +452,24 @@ void fill_post_wcc(struct svc_fh *fhp)
> >>>> nfs3svc_decode_symlinkargs(struct svc_rqst *rqstp, __be32 *p)
> >>>> {
> >>>> 	struct nfsd3_symlinkargs *args = rqstp->rq_argp;
> >>>> -	unsigned int len, avail;
> >>>> -	char *old, *new;
> >>>> -	struct kvec *vec;
> >>>> +	char *base = (char *)p;
> >>>> +	size_t dlen;
> >>>> 
> >>>> 	if (!(p = decode_fh(p, &args->ffh)) ||
> >>>> -	    !(p = decode_filename(p, &args->fname, &args->flen))
> >>>> -		)
> >>>> +	    !(p = decode_filename(p, &args->fname, &args->flen)))
> >>>> 		return 0;
> >>>> 	p = decode_sattr3(p, &args->attrs);
> >>>> 
> >>>> -	/* now decode the pathname, which might be larger than the first page.
> >>>> -	 * As we have to check for nul's anyway, we copy it into a new page
> >>>> -	 * This page appears in the rq_res.pages list, but as pages_len is always
> >>>> -	 * 0, it won't get in the way
> >>>> -	 */
> >>>> -	len = ntohl(*p++);
> >>>> -	if (len == 0 || len > NFS3_MAXPATHLEN || len >= PAGE_SIZE)
> >>>> -		return 0;
> >>>> -	args->tname = new = page_address(*(rqstp->rq_next_page++));
> >>>> -	args->tlen = len;
> >>>> -	/* first copy and check from the first page */
> >>>> -	old = (char*)p;
> >>>> -	vec = &rqstp->rq_arg.head[0];
> >>>> -	if ((void *)old > vec->iov_base + vec->iov_len)
> >>>> -		return 0;
> >>>> -	avail = vec->iov_len - (old - (char*)vec->iov_base);
> >>>> -	while (len && avail && *old) {
> >>>> -		*new++ = *old++;
> >>>> -		len--;
> >>>> -		avail--;
> >>>> -	}
> >>>> -	/* now copy next page if there is one */
> >>>> -	if (len && !avail && rqstp->rq_arg.page_len) {
> >>>> -		avail = min_t(unsigned int, rqstp->rq_arg.page_len, PAGE_SIZE);
> >>>> -		old = page_address(rqstp->rq_arg.pages[0]);
> >>>> -	}
> >>>> -	while (len && avail && *old) {
> >>>> -		*new++ = *old++;
> >>>> -		len--;
> >>>> -		avail--;
> >>>> -	}
> >>>> -	*new = '\0';
> >>>> -	if (len)
> >>>> -		return 0;
> >>>> +	args->tlen = ntohl(*p++);
> >>>> 
> >>>> +	args->first.iov_base = p;
> >>>> +	args->first.iov_len = rqstp->rq_arg.head[0].iov_len;
> >>>> +	args->first.iov_len -= (char *)p - base;
> >>>> +
> >>>> +	dlen = args->first.iov_len + rqstp->rq_arg.page_len +
> >>>> +	       rqstp->rq_arg.tail[0].iov_len;
> >>>> +	if (dlen < XDR_QUADLEN(args->tlen) << 2)
> >>>> +		return 0;
> >>>> 	return 1;
> >>>> }
> >>>> 
> >>>> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> >>>> index 5029b96..36bd1f7 100644
> >>>> --- a/fs/nfsd/nfs4proc.c
> >>>> +++ b/fs/nfsd/nfs4proc.c
> >>>> @@ -605,6 +605,13 @@ static void gen_boot_verifier(nfs4_verifier *verifier, struct net *net)
> >>>> 
> >>>> 	switch (create->cr_type) {
> >>>> 	case NF4LNK:
> >>>> +		if (create->cr_datalen > NFS4_MAXPATHLEN)
> >>>> +			return nfserr_nametoolong;
> >>>> +		create->cr_data =
> >>>> +			svc_fill_symlink_pathname(rqstp, &create->cr_first,
> >>>> +						  create->cr_datalen);
> >>>> +		if (IS_ERR(create->cr_data))
> >>>> +			return nfserrno(PTR_ERR(create->cr_data));
> >>>> 		status = nfsd_symlink(rqstp, &cstate->current_fh,
> >>>> 				      create->cr_name, create->cr_namelen,
> >>>> 				      create->cr_data, &resfh);
> >>>> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> >>>> index bd25230..d05384e 100644
> >>>> --- a/fs/nfsd/nfs4xdr.c
> >>>> +++ b/fs/nfsd/nfs4xdr.c
> >>>> @@ -648,6 +648,7 @@ static __be32 nfsd4_decode_bind_conn_to_session(struct nfsd4_compoundargs *argp,
> >>>> static __be32
> >>>> nfsd4_decode_create(struct nfsd4_compoundargs *argp, struct nfsd4_create *create)
> >>>> {
> >>>> +	struct kvec *head;
> >>>> 	DECODE_HEAD;
> >>>> 
> >>>> 	READ_BUF(4);
> >>>> @@ -656,10 +657,13 @@ static __be32 nfsd4_decode_bind_conn_to_session(struct nfsd4_compoundargs *argp,
> >>>> 	case NF4LNK:
> >>>> 		READ_BUF(4);
> >>>> 		create->cr_datalen = be32_to_cpup(p++);
> >>>> +		if (create->cr_datalen == 0)
> >>>> +			return nfserr_inval;
> >>>> +		head = argp->rqstp->rq_arg.head;
> >>>> +		create->cr_first.iov_base = p;
> >>>> +		create->cr_first.iov_len = head->iov_len;
> >>>> +		create->cr_first.iov_len -= (char *)p - (char *)head->iov_base;
> >>>> 		READ_BUF(create->cr_datalen);
> >>>> -		create->cr_data = svcxdr_dupstr(argp, p, create->cr_datalen);
> >>>> -		if (!create->cr_data)
> >>>> -			return nfserr_jukebox;
> >>>> 		break;
> >>>> 	case NF4BLK:
> >>>> 	case NF4CHR:
> >>>> diff --git a/fs/nfsd/nfsproc.c b/fs/nfsd/nfsproc.c
> >>>> index 1995ea6..f107f9f 100644
> >>>> --- a/fs/nfsd/nfsproc.c
> >>>> +++ b/fs/nfsd/nfsproc.c
> >>>> @@ -449,17 +449,19 @@
> >>>> 	struct svc_fh	newfh;
> >>>> 	__be32		nfserr;
> >>>> 
> >>>> +	if (argp->tlen > NFS_MAXPATHLEN)
> >>>> +		return nfserr_nametoolong;
> >>>> +
> >>>> +	argp->tname = svc_fill_symlink_pathname(rqstp, &argp->first,
> >>>> +						argp->tlen);
> >>>> +	if (IS_ERR(argp->tname))
> >>>> +		return nfserrno(PTR_ERR(argp->tname));
> >>>> +
> >>>> 	dprintk("nfsd: SYMLINK  %s %.*s -> %.*s\n",
> >>>> 		SVCFH_fmt(&argp->ffh), argp->flen, argp->fname,
> >>>> 		argp->tlen, argp->tname);
> >>>> 
> >>>> 	fh_init(&newfh, NFS_FHSIZE);
> >>>> -	/*
> >>>> -	 * Crazy hack: the request fits in a page, and already-decoded
> >>>> -	 * attributes follow argp->tname, so it's safe to just write a
> >>>> -	 * null to ensure it's null-terminated:
> >>>> -	 */
> >>>> -	argp->tname[argp->tlen] = '\0';
> >>>> 	nfserr = nfsd_symlink(rqstp, &argp->ffh, argp->fname, argp->flen,
> >>>> 						 argp->tname, &newfh);
> >>>> 
> >>>> diff --git a/fs/nfsd/nfsxdr.c b/fs/nfsd/nfsxdr.c
> >>>> index 165e25e..8fcd047 100644
> >>>> --- a/fs/nfsd/nfsxdr.c
> >>>> +++ b/fs/nfsd/nfsxdr.c
> >>>> @@ -71,22 +71,6 @@ __be32 *nfs2svc_decode_fh(__be32 *p, struct svc_fh *fhp)
> >>>> }
> >>>> 
> >>>> static __be32 *
> >>>> -decode_pathname(__be32 *p, char **namp, unsigned int *lenp)
> >>>> -{
> >>>> -	char		*name;
> >>>> -	unsigned int	i;
> >>>> -
> >>>> -	if ((p = xdr_decode_string_inplace(p, namp, lenp, NFS_MAXPATHLEN)) != NULL) {
> >>>> -		for (i = 0, name = *namp; i < *lenp; i++, name++) {
> >>>> -			if (*name == '\0')
> >>>> -				return NULL;
> >>>> -		}
> >>>> -	}
> >>>> -
> >>>> -	return p;
> >>>> -}
> >>>> -
> >>>> -static __be32 *
> >>>> decode_sattr(__be32 *p, struct iattr *iap)
> >>>> {
> >>>> 	u32	tmp, tmp1;
> >>>> @@ -383,14 +367,39 @@ __be32 *nfs2svc_encode_fattr(struct svc_rqst *rqstp, __be32 *p, struct svc_fh *f
> >>>> nfssvc_decode_symlinkargs(struct svc_rqst *rqstp, __be32 *p)
> >>>> {
> >>>> 	struct nfsd_symlinkargs *args = rqstp->rq_argp;
> >>>> +	char *base = (char *)p;
> >>>> +	size_t xdrlen;
> >>>> 
> >>>> 	if (   !(p = decode_fh(p, &args->ffh))
> >>>> -	    || !(p = decode_filename(p, &args->fname, &args->flen))
> >>>> -	    || !(p = decode_pathname(p, &args->tname, &args->tlen)))
> >>>> +	    || !(p = decode_filename(p, &args->fname, &args->flen)))
> >>>> 		return 0;
> >>>> -	p = decode_sattr(p, &args->attrs);
> >>>> 
> >>>> -	return xdr_argsize_check(rqstp, p);
> >>>> +	args->tlen = ntohl(*p++);
> >>>> +	if (args->tlen == 0)
> >>>> +		return 0;
> >>>> +
> >>>> +	args->first.iov_base = p;
> >>>> +	args->first.iov_len = rqstp->rq_arg.head[0].iov_len;
> >>>> +	args->first.iov_len -= (char *)p - base;
> >>>> +
> >>>> +	/* This request is never larger than a page. Therefore,
> >>>> +	 * transport will deliver either:
> >>>> +	 * 1. pathname in the pagelist -> sattr is in the tail.
> >>>> +	 * 2. everything in the head buffer -> sattr is in the head.
> >>>> +	 */
> >>>> +	if (rqstp->rq_arg.page_len) {
> >>>> +		if (args->tlen != rqstp->rq_arg.page_len)
> >>>> +			return 0;
> >>>> +		p = rqstp->rq_arg.tail[0].iov_base;
> >>>> +	} else {
> >>>> +		xdrlen = XDR_QUADLEN(args->tlen);
> >>>> +		if (xdrlen > args->first.iov_len - (8 * sizeof(__be32)))
> >>>> +			return 0;
> >>>> +		p += xdrlen;
> >>>> +	}
> >>>> +	decode_sattr(p, &args->attrs);
> >>>> +
> >>>> +	return 1;
> >>>> }
> >>>> 
> >>>> int
> >>>> diff --git a/fs/nfsd/xdr.h b/fs/nfsd/xdr.h
> >>>> index a765c41..ea7cca3 100644
> >>>> --- a/fs/nfsd/xdr.h
> >>>> +++ b/fs/nfsd/xdr.h
> >>>> @@ -72,6 +72,7 @@ struct nfsd_symlinkargs {
> >>>> 	char *			tname;
> >>>> 	unsigned int		tlen;
> >>>> 	struct iattr		attrs;
> >>>> +	struct kvec		first;
> >>>> };
> >>>> 
> >>>> struct nfsd_readdirargs {
> >>>> diff --git a/fs/nfsd/xdr3.h b/fs/nfsd/xdr3.h
> >>>> index deccf7f..2cb29e9 100644
> >>>> --- a/fs/nfsd/xdr3.h
> >>>> +++ b/fs/nfsd/xdr3.h
> >>>> @@ -90,6 +90,7 @@ struct nfsd3_symlinkargs {
> >>>> 	char *			tname;
> >>>> 	unsigned int		tlen;
> >>>> 	struct iattr		attrs;
> >>>> +	struct kvec		first;
> >>>> };
> >>>> 
> >>>> struct nfsd3_readdirargs {
> >>>> diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h
> >>>> index d56219d..b485cd1 100644
> >>>> --- a/fs/nfsd/xdr4.h
> >>>> +++ b/fs/nfsd/xdr4.h
> >>>> @@ -110,6 +110,7 @@ struct nfsd4_create {
> >>>> 		struct {
> >>>> 			u32 datalen;
> >>>> 			char *data;
> >>>> +			struct kvec first;
> >>>> 		} link;   /* NF4LNK */
> >>>> 		struct {
> >>>> 			u32 specdata1;
> >>>> @@ -124,6 +125,7 @@ struct nfsd4_create {
> >>>> };
> >>>> #define cr_datalen	u.link.datalen
> >>>> #define cr_data		u.link.data
> >>>> +#define cr_first	u.link.first
> >>>> #define cr_specdata1	u.dev.specdata1
> >>>> #define cr_specdata2	u.dev.specdata2
> >>>> 
> >>>> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> >>>> index 238b9ae..fd5846e 100644
> >>>> --- a/include/linux/sunrpc/svc.h
> >>>> +++ b/include/linux/sunrpc/svc.h
> >>>> @@ -495,6 +495,8 @@ int		   svc_register(const struct svc_serv *, struct net *, const int,
> >>>> char *		   svc_print_addr(struct svc_rqst *, char *, size_t);
> >>>> unsigned int	   svc_fill_write_vector(struct svc_rqst *rqstp,
> >>>> 					 struct kvec *first, size_t total);
> >>>> +char		  *svc_fill_symlink_pathname(struct svc_rqst *rqstp,
> >>>> +					     struct kvec *first, size_t total);
> >>>> 
> >>>> #define	RPC_MAX_ADDRBUFLEN	(63U)
> >>>> 
> >>>> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> >>>> index 759b668..fc93406 100644
> >>>> --- a/net/sunrpc/svc.c
> >>>> +++ b/net/sunrpc/svc.c
> >>>> @@ -1578,3 +1578,70 @@ unsigned int svc_fill_write_vector(struct svc_rqst *rqstp, struct kvec *first,
> >>>> 	return i;
> >>>> }
> >>>> EXPORT_SYMBOL_GPL(svc_fill_write_vector);
> >>>> +
> >>>> +/**
> >>>> + * svc_fill_symlink_pathname - Construct pathname argument for VFS symlink call
> >>>> + * @rqstp: svc_rqst to operate on
> >>>> + * @first: buffer containing first section of pathname
> >>>> + * @total: total length of the pathname argument
> >>>> + *
> >>>> + * Returns pointer to a NUL-terminated string, or an ERR_PTR. The buffer is
> >>>> + * released automatically when @rqstp is recycled.
> >>>> + */
> >>>> +char *svc_fill_symlink_pathname(struct svc_rqst *rqstp, struct kvec *first,
> >>>> +				size_t total)
> >>>> +{
> >>>> +	struct xdr_buf *arg = &rqstp->rq_arg;
> >>>> +	struct page **pages;
> >>>> +	char *result;
> >>>> +
> >>>> +	/* VFS API demands a NUL-terminated pathname. This function
> >>>> +	 * uses a page from @rqstp as the pathname buffer, to enable
> >>>> +	 * direct placement. Thus the total buffer size is PAGE_SIZE.
> >>>> +	 * Space in this buffer for NUL-termination requires that we
> >>>> +	 * cap the size of the returned symlink pathname just a
> >>>> +	 * little early.
> >>>> +	 */
> >>>> +	if (total > PAGE_SIZE - 1)
> >>>> +		return ERR_PTR(-ENAMETOOLONG);
> >>>> +
> >>>> +	/* Some types of transport can present the pathname entirely
> >>>> +	 * in rq_arg.pages. If not, then copy the pathname into one
> >>>> +	 * page.
> >>>> +	 */
> >>>> +	pages = arg->pages;
> >>>> +	WARN_ON_ONCE(arg->page_base != 0);
> >>>> +	if (first->iov_base == 0) {
> >>>> +		result = page_address(*pages);
> >>>> +		result[total] = '\0';
> >>>> +	} else {
> >>>> +		size_t len, remaining;
> >>>> +		char *dst;
> >>>> +
> >>>> +		result = page_address(*(rqstp->rq_next_page++));
> >>>> +		dst = result;
> >>>> +		remaining = total;
> >>>> +
> >>>> +		len = min_t(size_t, total, first->iov_len);
> >>>> +		memcpy(dst, first->iov_base, len);
> >>>> +		dst += len;
> >>>> +		remaining -= len;
> >>>> +
> >>>> +		/* No more than one page left */
> >>>> +		if (remaining) {
> >>>> +			len = min_t(size_t, remaining, PAGE_SIZE);
> >>>> +			memcpy(dst, page_address(*pages), len);
> >>>> +			dst += len;
> >>>> +		}
> >>>> +
> >>>> +		*dst = '\0';
> >>>> +	}
> >>>> +
> >>>> +	/* Sanity check: we don't allow the pathname argument to
> >>>> +	 * contain a NUL byte.
> >>>> +	 */
> >>>> +	if (strlen(result) != total)
> >>>> +		return ERR_PTR(-EINVAL);
> >>>> +	return result;
> >>>> +}
> >>>> +EXPORT_SYMBOL_GPL(svc_fill_symlink_pathname);
> >> 
> >> --
> >> Chuck Lever
> 
> --
> Chuck Lever
> 
> 
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