[PATCH v4 44/68] cachefiles: Implement key to filename encoding
Jeff Layton
jlayton at kernel.org
Thu Jan 6 09:43:37 PST 2022
On Wed, 2021-12-22 at 23:24 +0000, David Howells wrote:
> Implement a function to encode a binary cookie key as something that can be
> used as a filename. Four options are considered:
>
> (1) All printable chars with no '/' characters. Prepend a 'D' to indicate
> the encoding but otherwise use as-is.
>
> (2) Appears to be an array of __be32. Encode as 'S' plus a list of
> hex-encoded 32-bit ints separated by commas. If a number is 0, it is
> rendered as "" instead of "0".
>
> (3) Appears to be an array of __le32. Encoded as (2) but with a 'T'
> encoding prefix.
>
> (4) Encoded as base64 with an 'E' prefix plus a second char indicating how
> much padding is involved. A non-standard base64 encoding is used
> because '/' cannot be used in the encoded form.
>
> If (1) is not possible, whichever of (2), (3) or (4) produces the shortest
> string is selected (hex-encoding a number may be less dense than base64
> encoding it).
>
Since most cookies are fairly small, is there any real benefit to
optimizing for length here? How much inflation are we talking about?
> Note that the prefix characters have to be selected from the set [DEIJST@]
> lest cachefilesd remove the files because it recognise the name.
>
> Changes
> =======
> ver #2:
> - Fix a short allocation that didn't allow for a string terminator[1]
>
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: linux-cachefs at redhat.com
> Link: https://lore.kernel.org/r/bcefb8f2-576a-b3fc-cc29-89808ebfd7c1@linux.alibaba.com/ [1]
> Link: https://lore.kernel.org/r/163819640393.215744.15212364106412961104.stgit@warthog.procyon.org.uk/ # v1
> Link: https://lore.kernel.org/r/163906940529.143852.17352132319136117053.stgit@warthog.procyon.org.uk/ # v2
> Link: https://lore.kernel.org/r/163967149827.1823006.6088580775428487961.stgit@warthog.procyon.org.uk/ # v3
> ---
>
> fs/cachefiles/Makefile | 1
> fs/cachefiles/internal.h | 5 ++
> fs/cachefiles/key.c | 138 ++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 144 insertions(+)
> create mode 100644 fs/cachefiles/key.c
>
> diff --git a/fs/cachefiles/Makefile b/fs/cachefiles/Makefile
> index d67210ece9cd..6f025940a65c 100644
> --- a/fs/cachefiles/Makefile
> +++ b/fs/cachefiles/Makefile
> @@ -7,6 +7,7 @@ cachefiles-y := \
> cache.o \
> daemon.o \
> interface.o \
> + key.o \
> main.o \
> namei.o \
> security.o \
> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
> index 8763ee4a0df2..dbc37f5d4714 100644
> --- a/fs/cachefiles/internal.h
> +++ b/fs/cachefiles/internal.h
> @@ -173,6 +173,11 @@ extern struct cachefiles_object *cachefiles_grab_object(struct cachefiles_object
> extern void cachefiles_put_object(struct cachefiles_object *object,
> enum cachefiles_obj_ref_trace why);
>
> +/*
> + * key.c
> + */
> +extern bool cachefiles_cook_key(struct cachefiles_object *object);
> +
> /*
> * main.c
> */
> diff --git a/fs/cachefiles/key.c b/fs/cachefiles/key.c
> new file mode 100644
> index 000000000000..bf935e25bdbe
> --- /dev/null
> +++ b/fs/cachefiles/key.c
> @@ -0,0 +1,138 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/* Key to pathname encoder
> + *
> + * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells at redhat.com)
> + */
> +
> +#include <linux/slab.h>
> +#include "internal.h"
> +
> +static const char cachefiles_charmap[64] =
> + "0123456789" /* 0 - 9 */
> + "abcdefghijklmnopqrstuvwxyz" /* 10 - 35 */
> + "ABCDEFGHIJKLMNOPQRSTUVWXYZ" /* 36 - 61 */
> + "_-" /* 62 - 63 */
> + ;
> +
> +static const char cachefiles_filecharmap[256] = {
> + /* we skip space and tab and control chars */
> + [33 ... 46] = 1, /* '!' -> '.' */
> + /* we skip '/' as it's significant to pathwalk */
> + [48 ... 127] = 1, /* '0' -> '~' */
> +};
> +
> +static inline unsigned int how_many_hex_digits(unsigned int x)
> +{
> + return x ? round_up(ilog2(x) + 1, 4) / 4 : 0;
> +}
> +
> +/*
> + * turn the raw key into something cooked
> + * - the key may be up to NAME_MAX in length (including the length word)
> + * - "base64" encode the strange keys, mapping 3 bytes of raw to four of
> + * cooked
> + * - need to cut the cooked key into 252 char lengths (189 raw bytes)
> + */
> +bool cachefiles_cook_key(struct cachefiles_object *object)
> +{
> + const u8 *key = fscache_get_key(object->cookie), *kend;
> + unsigned char ch;
> + unsigned int acc, i, n, nle, nbe, keylen = object->cookie->key_len;
> + unsigned int b64len, len, print, pad;
> + char *name, sep;
> +
> + _enter(",%u,%*phN", keylen, keylen, key);
> +
> + BUG_ON(keylen > NAME_MAX - 3);
> +
> + print = 1;
> + for (i = 0; i < keylen; i++) {
> + ch = key[i];
> + print &= cachefiles_filecharmap[ch];
> + }
> +
> + /* If the path is usable ASCII, then we render it directly */
> + if (print) {
> + len = 1 + keylen;
> + name = kmalloc(len + 1, GFP_KERNEL);
> + if (!name)
> + return false;
> +
> + name[0] = 'D'; /* Data object type, string encoding */
> + memcpy(name + 1, key, keylen);
> + goto success;
> + }
> +
> + /* See if it makes sense to encode it as "hex,hex,hex" for each 32-bit
> + * chunk. We rely on the key having been padded out to a whole number
> + * of 32-bit words.
> + */
> + n = round_up(keylen, 4);
> + nbe = nle = 0;
> + for (i = 0; i < n; i += 4) {
> + u32 be = be32_to_cpu(*(__be32 *)(key + i));
> + u32 le = le32_to_cpu(*(__le32 *)(key + i));
> +
> + nbe += 1 + how_many_hex_digits(be);
> + nle += 1 + how_many_hex_digits(le);
> + }
> +
> + b64len = DIV_ROUND_UP(keylen, 3);
> + pad = b64len * 3 - keylen;
> + b64len = 2 + b64len * 4; /* Length if we base64-encode it */
> + _debug("len=%u nbe=%u nle=%u b64=%u", keylen, nbe, nle, b64len);
> + if (nbe < b64len || nle < b64len) {
> + unsigned int nlen = min(nbe, nle) + 1;
> + name = kmalloc(nlen, GFP_KERNEL);
> + if (!name)
> + return false;
> + sep = (nbe <= nle) ? 'S' : 'T'; /* Encoding indicator */
> + len = 0;
> + for (i = 0; i < n; i += 4) {
> + u32 x;
> + if (nbe <= nle)
> + x = be32_to_cpu(*(__be32 *)(key + i));
> + else
> + x = le32_to_cpu(*(__le32 *)(key + i));
> + name[len++] = sep;
> + if (x != 0)
> + len += snprintf(name + len, nlen - len, "%x", x);
> + sep = ',';
> + }
> + goto success;
> + }
> +
> + /* We need to base64-encode it */
> + name = kmalloc(b64len + 1, GFP_KERNEL);
> + if (!name)
> + return false;
> +
> + name[0] = 'E';
> + name[1] = '0' + pad;
> + len = 2;
> + kend = key + keylen;
> + do {
> + acc = *key++;
> + if (key < kend) {
> + acc |= *key++ << 8;
> + if (key < kend)
> + acc |= *key++ << 16;
> + }
> +
> + name[len++] = cachefiles_charmap[acc & 63];
> + acc >>= 6;
> + name[len++] = cachefiles_charmap[acc & 63];
> + acc >>= 6;
> + name[len++] = cachefiles_charmap[acc & 63];
> + acc >>= 6;
> + name[len++] = cachefiles_charmap[acc & 63];
> + } while (key < kend);
It might be good to eventually consolidate this code with the base64
scheme that fscrypt uses. Are they compatible? If so, then that can be
done in a later merge.
> +
> +success:
> + name[len] = 0;
> + object->d_name = name;
> + object->d_name_len = len;
> + _leave(" = %s", object->d_name);
> + return true;
> +}
>
>
--
Jeff Layton <jlayton at kernel.org>
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