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SubjectRe: [PATCH v3 1/2] ceph: fix buffer length handling in virtual xattrs
On Mon, Jun 24, 2019 at 12:26 PM Jeff Layton <jlayton@kernel.org> wrote:
>
> On Mon, 2019-06-24 at 12:00 +0200, Ilya Dryomov wrote:
> > On Fri, Jun 21, 2019 at 4:18 PM Jeff Layton <jlayton@kernel.org> wrote:
> > > The convention with xattrs is to not store the termination with string
> > > data, given that it returns the length. This is how setfattr/getfattr
> > > operate.
> > >
> > > Most of ceph's virtual xattr routines use snprintf to plop the string
> > > directly into the destination buffer, but snprintf always NULL
> > > terminates the string. This means that if we send the kernel a buffer
> > > that is the exact length needed to hold the string, it'll end up
> > > truncated.
> > >
> > > Add new routines to format the string into an on-stack buffer that is
> > > always large enough to hold the whole thing and then memcpy the result
> > > into the destination buffer. Then, change over the virtual xattr
> > > routines to use the new helper functions as appropriate.
> > >
> > > Finally, make the code return ERANGE if the destination buffer size was
> > > too small to hold the returned value.
> > >
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > > fs/ceph/xattr.c | 103 ++++++++++++++++++++++++++++++++++++------------
> > > 1 file changed, 78 insertions(+), 25 deletions(-)
> > >
> > > diff --git a/fs/ceph/xattr.c b/fs/ceph/xattr.c
> > > index 6621d27e64f5..359d3cbbb37b 100644
> > > --- a/fs/ceph/xattr.c
> > > +++ b/fs/ceph/xattr.c
> > > @@ -112,22 +112,47 @@ static size_t ceph_vxattrcb_layout(struct ceph_inode_info *ci, char *val,
> > > return ret;
> > > }
> > >
> > > +/* Enough to hold any possible expression of integer TYPE in base 10 */
> > > +#define INT_STR_SIZE(_type) 3*sizeof(_type)+2
> > > +
> > > +/*
> > > + * snprintf always NULL terminates, but we need for xattrs to not be. For
> > > + * the integer vxattrs, just create an on-stack buffer for snprintf's
> > > + * destination, and just don't copy the termination to the actual buffer.
> > > + */
> > > +#define GENERATE_XATTR_INT_FORMATTER(_lbl, _format, _type) \
> > > +static size_t format_ ## _lbl ## _xattr(char *val, size_t size, _type src) \
> > > +{ \
> > > + size_t ret; \
> > > + char buf[INT_STR_SIZE(_type)]; \
> > > + \
> > > + ret = snprintf(buf, size ? sizeof(buf) : 0, _format, src); \
> > > + if (ret <= size) \
> > > + memcpy(val, buf, ret); \
> > > + return ret; \
> > > +}
> > > +
> > > +GENERATE_XATTR_INT_FORMATTER(u, "%u", unsigned int)
> > > +GENERATE_XATTR_INT_FORMATTER(d, "%d", int)
> > > +GENERATE_XATTR_INT_FORMATTER(lld, "%lld", long long)
> > > +GENERATE_XATTR_INT_FORMATTER(llu, "%llu", unsigned long long)
> > > +
> > > static size_t ceph_vxattrcb_layout_stripe_unit(struct ceph_inode_info *ci,
> > > char *val, size_t size)
> > > {
> > > - return snprintf(val, size, "%u", ci->i_layout.stripe_unit);
> > > + return format_u_xattr(val, size, ci->i_layout.stripe_unit);
> > > }
> > >
> > > static size_t ceph_vxattrcb_layout_stripe_count(struct ceph_inode_info *ci,
> > > char *val, size_t size)
> > > {
> > > - return snprintf(val, size, "%u", ci->i_layout.stripe_count);
> > > + return format_u_xattr(val, size, ci->i_layout.stripe_count);
> > > }
> > >
> > > static size_t ceph_vxattrcb_layout_object_size(struct ceph_inode_info *ci,
> > > char *val, size_t size)
> > > {
> > > - return snprintf(val, size, "%u", ci->i_layout.object_size);
> > > + return format_u_xattr(val, size, ci->i_layout.object_size);
> > > }
> > >
> > > static size_t ceph_vxattrcb_layout_pool(struct ceph_inode_info *ci,
> > > @@ -141,10 +166,14 @@ static size_t ceph_vxattrcb_layout_pool(struct ceph_inode_info *ci,
> > >
> > > down_read(&osdc->lock);
> > > pool_name = ceph_pg_pool_name_by_id(osdc->osdmap, pool);
> > > - if (pool_name)
> > > - ret = snprintf(val, size, "%s", pool_name);
> > > - else
> > > - ret = snprintf(val, size, "%lld", (unsigned long long)pool);
> > > + if (pool_name) {
> > > + ret = strlen(pool_name);
> > > +
> > > + if (ret <= size)
> > > + memcpy(val, pool_name, ret);
> > > + } else {
> > > + ret = format_lld_xattr(val, size, pool);
> > > + }
> > > up_read(&osdc->lock);
> > > return ret;
> > > }
> > > @@ -155,7 +184,11 @@ static size_t ceph_vxattrcb_layout_pool_namespace(struct ceph_inode_info *ci,
> > > int ret = 0;
> > > struct ceph_string *ns = ceph_try_get_string(ci->i_layout.pool_ns);
> > > if (ns) {
> > > - ret = snprintf(val, size, "%.*s", (int)ns->len, ns->str);
> > > + ret = ns->len;
> > > +
> > > + if (ret <= size)
> > > + memcpy(val, ns->str, ns->len);
> > > +
> > > ceph_put_string(ns);
> > > }
> > > return ret;
> > > @@ -166,50 +199,61 @@ static size_t ceph_vxattrcb_layout_pool_namespace(struct ceph_inode_info *ci,
> > > static size_t ceph_vxattrcb_dir_entries(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_files + ci->i_subdirs);
> > > + return format_lld_xattr(val, size, ci->i_files + ci->i_subdirs);
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_files(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_files);
> > > + return format_lld_xattr(val, size, ci->i_files);
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_subdirs(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_subdirs);
> > > + return format_lld_xattr(val, size, ci->i_subdirs);
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_rentries(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_rfiles + ci->i_rsubdirs);
> > > + return format_lld_xattr(val, size, ci->i_rfiles + ci->i_rsubdirs);
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_rfiles(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_rfiles);
> > > + return format_lld_xattr(val, size, ci->i_rfiles);
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_rsubdirs(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_rsubdirs);
> > > + return format_lld_xattr(val, size, ci->i_rsubdirs);
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_rbytes(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld", ci->i_rbytes);
> > > + return format_lld_xattr(val, size, ci->i_rbytes);
> > > +}
> > > +
> > > +static size_t format_ts64_xattr(char *val, size_t size, struct timespec64 *src)
> > > +{
> > > + size_t ret;
> > > + char buf[INT_STR_SIZE(long long) + 1 + 9];
> > > +
> > > + ret = snprintf(buf, size ? sizeof(buf) : 0, "%lld.%09ld", src->tv_sec,
> > > + src->tv_nsec);
> > > + if (ret <= size)
> > > + memcpy(val, buf, ret);
> > > + return ret;
> > > }
> > >
> > > static size_t ceph_vxattrcb_dir_rctime(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld.%09ld", ci->i_rctime.tv_sec,
> > > - ci->i_rctime.tv_nsec);
> > > + return format_ts64_xattr(val, size, &ci->i_rctime);
> > > }
> > >
> > > /* dir pin */
> > > @@ -221,7 +265,7 @@ static bool ceph_vxattrcb_dir_pin_exists(struct ceph_inode_info *ci)
> > > static size_t ceph_vxattrcb_dir_pin(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%d", (int)ci->i_dir_pin);
> > > + return format_d_xattr(val, size, ci->i_dir_pin);
> > > }
> > >
> > > /* quotas */
> > > @@ -241,20 +285,27 @@ static bool ceph_vxattrcb_quota_exists(struct ceph_inode_info *ci)
> > > static size_t ceph_vxattrcb_quota(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "max_bytes=%llu max_files=%llu",
> > > - ci->i_max_bytes, ci->i_max_files);
> > > + size_t ret;
> > > + char buf[(2*INT_STR_SIZE(unsigned long long)) + 10 + 11];
> > > +
> > > + ret = snprintf(buf, size ? sizeof(buf) : 0,
> > > + "max_bytes=%llu max_files=%llu",
> > > + ci->i_max_bytes, ci->i_max_files);
> > > + if (ret <= size)
> > > + memcpy(val, buf, ret);
> > > + return ret;
> > > }
> > >
> > > static size_t ceph_vxattrcb_quota_max_bytes(struct ceph_inode_info *ci,
> > > char *val, size_t size)
> > > {
> > > - return snprintf(val, size, "%llu", ci->i_max_bytes);
> > > + return format_llu_xattr(val, size, ci->i_max_bytes);
> > > }
> > >
> > > static size_t ceph_vxattrcb_quota_max_files(struct ceph_inode_info *ci,
> > > char *val, size_t size)
> > > {
> > > - return snprintf(val, size, "%llu", ci->i_max_files);
> > > + return format_llu_xattr(val, size, ci->i_max_files);
> > > }
> > >
> > > /* snapshots */
> > > @@ -266,8 +317,7 @@ static bool ceph_vxattrcb_snap_btime_exists(struct ceph_inode_info *ci)
> > > static size_t ceph_vxattrcb_snap_btime(struct ceph_inode_info *ci, char *val,
> > > size_t size)
> > > {
> > > - return snprintf(val, size, "%lld.%09ld", ci->i_snap_btime.tv_sec,
> > > - ci->i_snap_btime.tv_nsec);
> > > + return format_ts64_xattr(val, size, &ci->i_snap_btime);
> > > }
> > >
> > > #define CEPH_XATTR_NAME(_type, _name) XATTR_CEPH_PREFIX #_type "." #_name
> >
> > Hi Jeff,
> >
> > This seems over-engineered to me. You have four functions just for
> > ints, two more for ts64 and quota and several ad-hoc %s memcpys. Why
> > not define a single function with a generously-sized buffer, BUG in
> > case it's too small and take a format string?
>
> Having to declare a 64 byte (or 128 byte or whatever) buffer on the
> stack for an int seemed a bit wasteful, but ok. We can certainly do it
> that way instead.

This is a leaf function, unlikely to be called deep in the stack.
I would be fine with pretty much any (reasonable) buffer here.

Treating all xattrs the same is actually good IMO. A hypothetical
situation where printing e.g. pin succeeds but printing quota overflows
the stack would seem rather weird.

Thanks,

Ilya

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