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@xxxxxxxxxx> > cc: linux-cachefs@xxxxxxxxxx > Link: https://lore.kernel.org/r/bcefb8f2-576a-b3fc-cc29-89808ebfd7c1@xxxxxxxxxxxxxxxxx/ [1] > Link: https://lore.kernel.org/r/163819640393.215744.15212364106412961104.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1 > Link: https://lore.kernel.org/r/163906940529.143852.17352132319136117053.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2 > Link: https://lore.kernel.org/r/163967149827.1823006.6088580775428487961.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # 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@xxxxxxxxxx) > + */ > + > +#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@xxxxxxxxxx>
