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Date: Wed, 19 Feb 2020 21:07:11 +0100
From: Jann Horn <jannh@...gle.com>
To: David Howells <dhowells@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
Miklos Szeredi <mszeredi@...hat.com>,
Christian Brauner <christian@...uner.io>,
Linux API <linux-api@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/19] vfs: syscall: Add fsinfo() to query filesystem
information [ver #16]
On Tue, Feb 18, 2020 at 6:05 PM David Howells <dhowells@...hat.com> wrote:
> Add a system call to allow filesystem information to be queried. A request
> value can be given to indicate the desired attribute. Support is provided
> for enumerating multi-value attributes.
[...]
> +static const struct fsinfo_attribute fsinfo_common_attributes[];
> +
> +/**
> + * fsinfo_string - Store a string as an fsinfo attribute value.
> + * @s: The string to store (may be NULL)
> + * @ctx: The parameter context
> + */
> +int fsinfo_string(const char *s, struct fsinfo_context *ctx)
> +{
> + int ret = 0;
> +
> + if (s) {
> + ret = strlen(s);
> + memcpy(ctx->buffer, s, ret);
> + }
> +
> + return ret;
> +}
Please add a check here to ensure that "ret" actually fits into the
buffer (and use WARN_ON() if you think the check should never fire).
Otherwise I think this is too fragile.
[...]
> +static int fsinfo_generic_ids(struct path *path, struct fsinfo_context *ctx)
> +{
> + struct fsinfo_ids *p = ctx->buffer;
> + struct super_block *sb;
> + struct kstatfs buf;
> + int ret;
> +
> + ret = vfs_statfs(path, &buf);
> + if (ret < 0 && ret != -ENOSYS)
> + return ret;
What's going on here? If vfs_statfs() returns -ENOSYS, we just use the
(AFAICS uninitialized) buf.f_fsid anyway in the memcpy() below and
return it to userspace?
> + sb = path->dentry->d_sb;
> + p->f_fstype = sb->s_magic;
> + p->f_dev_major = MAJOR(sb->s_dev);
> + p->f_dev_minor = MINOR(sb->s_dev);
> +
> + memcpy(&p->f_fsid, &buf.f_fsid, sizeof(p->f_fsid));
> + strlcpy(p->f_fs_name, path->dentry->d_sb->s_type->name,
> + sizeof(p->f_fs_name));
> + return sizeof(*p);
> +}
[...]
> +static int fsinfo_attribute_info(struct path *path, struct fsinfo_context *ctx)
> +{
> + const struct fsinfo_attribute *attr;
> + struct fsinfo_attribute_info *info = ctx->buffer;
> + struct dentry *dentry = path->dentry;
> +
> + if (dentry->d_sb->s_op->fsinfo_attributes)
> + for (attr = dentry->d_sb->s_op->fsinfo_attributes; attr->get; attr++)
> + if (attr->attr_id == ctx->Nth)
> + goto found;
> + for (attr = fsinfo_common_attributes; attr->get; attr++)
> + if (attr->attr_id == ctx->Nth)
> + goto found;
> + return -ENODATA;
> +
> +found:
> + info->attr_id = attr->attr_id;
> + info->type = attr->type;
> + info->flags = attr->flags;
> + info->size = attr->size;
> + info->element_size = attr->element_size;
> + return sizeof(*attr);
I think you meant sizeof(*info).
[...]
> +static int fsinfo_attributes(struct path *path, struct fsinfo_context *ctx)
> +{
> + const struct fsinfo_attribute *attr;
> + struct super_block *sb = path->dentry->d_sb;
> +
> + if (sb->s_op->fsinfo_attributes)
> + for (attr = sb->s_op->fsinfo_attributes; attr->get; attr++)
> + fsinfo_attributes_insert(ctx, attr);
> + for (attr = fsinfo_common_attributes; attr->get; attr++)
> + fsinfo_attributes_insert(ctx, attr);
> + return ctx->usage;
It is kind of weird that you have to return the ctx->usage everywhere
even though the caller already has ctx...
> +}
> +
> +static const struct fsinfo_attribute fsinfo_common_attributes[] = {
> + FSINFO_VSTRUCT (FSINFO_ATTR_STATFS, fsinfo_generic_statfs),
> + FSINFO_VSTRUCT (FSINFO_ATTR_IDS, fsinfo_generic_ids),
> + FSINFO_VSTRUCT (FSINFO_ATTR_LIMITS, fsinfo_generic_limits),
> + FSINFO_VSTRUCT (FSINFO_ATTR_SUPPORTS, fsinfo_generic_supports),
> + FSINFO_VSTRUCT (FSINFO_ATTR_TIMESTAMP_INFO, fsinfo_generic_timestamp_info),
> + FSINFO_STRING (FSINFO_ATTR_VOLUME_ID, fsinfo_generic_volume_id),
> + FSINFO_VSTRUCT (FSINFO_ATTR_VOLUME_UUID, fsinfo_generic_volume_uuid),
> + FSINFO_VSTRUCT_N(FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO, fsinfo_attribute_info),
> + FSINFO_LIST (FSINFO_ATTR_FSINFO_ATTRIBUTES, fsinfo_attributes),
> + {}
> +};
> +
> +/*
> + * Retrieve large filesystem information, such as an opaque blob or array of
> + * struct elements where the value isn't limited to the size of a page.
> + */
> +static int vfs_fsinfo_large(struct path *path, struct fsinfo_context *ctx,
> + const struct fsinfo_attribute *attr)
> +{
> + int ret;
> +
> + while (!signal_pending(current)) {
> + ctx->usage = 0;
> + ret = attr->get(path, ctx);
> + if (IS_ERR_VALUE((long)ret))
> + return ret; /* Error */
> + if ((unsigned int)ret <= ctx->buf_size)
> + return ret; /* It fitted */
> +
> + /* We need to resize the buffer */
> + kvfree(ctx->buffer);
> + ctx->buffer = NULL;
> + ctx->buf_size = roundup(ret, PAGE_SIZE);
> + if (ctx->buf_size > INT_MAX)
> + return -EMSGSIZE;
> + ctx->buffer = kvmalloc(ctx->buf_size, GFP_KERNEL);
ctx->buffer is _almost_ always pre-zeroed (see vfs_do_fsinfo() below),
except if we have FSINFO_TYPE_OPAQUE or FSINFO_TYPE_LIST with a size
bigger than what the attribute's ->size field said? Is that
intentional?
> + if (!ctx->buffer)
> + return -ENOMEM;
> + }
> +
> + return -ERESTARTSYS;
> +}
> +
> +static int vfs_do_fsinfo(struct path *path, struct fsinfo_context *ctx,
> + const struct fsinfo_attribute *attr)
> +{
> + if (ctx->Nth != 0 && !(attr->flags & (FSINFO_FLAGS_N | FSINFO_FLAGS_NM)))
> + return -ENODATA;
> + if (ctx->Mth != 0 && !(attr->flags & FSINFO_FLAGS_NM))
> + return -ENODATA;
> +
> + ctx->buf_size = attr->size;
> + if (ctx->want_size_only && attr->type == FSINFO_TYPE_VSTRUCT)
> + return attr->size;
> +
> + ctx->buffer = kvzalloc(ctx->buf_size, GFP_KERNEL);
> + if (!ctx->buffer)
> + return -ENOMEM;
> +
> + switch (attr->type) {
> + case FSINFO_TYPE_VSTRUCT:
> + ctx->clear_tail = true;
> + /* Fall through */
> + case FSINFO_TYPE_STRING:
> + return attr->get(path, ctx);
> +
> + case FSINFO_TYPE_OPAQUE:
> + case FSINFO_TYPE_LIST:
> + return vfs_fsinfo_large(path, ctx, attr);
> +
> + default:
> + return -ENOPKG;
> + }
> +}
[...]
> +SYSCALL_DEFINE5(fsinfo,
> + int, dfd, const char __user *, pathname,
> + struct fsinfo_params __user *, params,
> + void __user *, user_buffer, size_t, user_buf_size)
> +{
[...]
> + if (ret < 0)
> + goto error;
> +
> + result_size = ret;
> + if (result_size > user_buf_size)
> + result_size = user_buf_size;
This is "result_size = min_t(size_t, ret, user_buf_size)".
[...]
> +/*
> + * A filesystem information attribute definition.
> + */
> +struct fsinfo_attribute {
> + unsigned int attr_id; /* The ID of the attribute */
> + enum fsinfo_value_type type:8; /* The type of the attribute's value(s) */
> + unsigned int flags:8;
> + unsigned int size:16; /* - Value size (FSINFO_STRUCT) */
> + unsigned int element_size:16; /* - Element size (FSINFO_LIST) */
> + int (*get)(struct path *path, struct fsinfo_context *params);
> +};
Why the bitfields? It doesn't look like that's going to help you much,
you'll just end up with 6 bytes of holes on x86-64:
$ cat fsinfo_attribute_layout.c
enum fsinfo_value_type {
FSINFO_TYPE_VSTRUCT = 0, /* Version-lengthed struct (up to
4096 bytes) */
FSINFO_TYPE_STRING = 1, /* NUL-term var-length string (up to
4095 chars) */
FSINFO_TYPE_OPAQUE = 2, /* Opaque blob (unlimited size) */
FSINFO_TYPE_LIST = 3, /* List of ints/structs (unlimited size) */
};
struct fsinfo_attribute {
unsigned int attr_id; /* The ID of the attribute */
enum fsinfo_value_type type:8; /* The type of the
attribute's value(s) */
unsigned int flags:8;
unsigned int size:16; /* - Value size (FSINFO_STRUCT) */
unsigned int element_size:16; /* - Element size (FSINFO_LIST) */
void *get;
};
void *blah(struct fsinfo_attribute *p) {
return p->get;
}
$ gcc -c -o fsinfo_attribute_layout.o fsinfo_attribute_layout.c -ggdb
$ pahole -C fsinfo_attribute -E --hex fsinfo_attribute_layout.o
struct fsinfo_attribute {
unsigned int attr_id; /* 0 0x4 */
enum fsinfo_value_type type:8; /* 0x4: 0 0x4 */
unsigned int flags:8; /* 0x4:0x8 0x4 */
unsigned int size:16; /* 0x4:0x10 0x4 */
unsigned int element_size:16; /* 0x8: 0 0x4 */
/* XXX 16 bits hole, try to pack */
/* XXX 4 bytes hole, try to pack */
void * get; /* 0x10 0x8 */
/* size: 24, cachelines: 1, members: 6 */
/* sum members: 12, holes: 1, sum holes: 4 */
/* sum bitfield members: 48 bits, bit holes: 1, sum bit holes:
16 bits */
/* last cacheline: 24 bytes */
};
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