Commit 5d439467 authored by David Howells's avatar David Howells
Browse files

cachefiles: Implement key to filename encoding 



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).

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: default avatarDavid Howells <dhowells@redhat.com>
Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
cc: linux-cachefs@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
Link: https://lore.kernel.org/r/164021549223.640689.14762875188193982341.stgit@warthog.procyon.org.uk/ # v4
parent df98e87f

fs/cachefiles/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -7,6 +7,7 @@ cachefiles-y := \ 
	cache.o \

	daemon.o \

	interface.o \

	key.o \

	main.o \

	namei.o \

	security.o \

fs/cachefiles/internal.h

+5 −0
Original line number Diff line number Diff line
@@ -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

 */

fs/cachefiles/key.c

0 → 100644
+138 −0
Original line number Diff line number Diff line 
// 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@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);



success:

	name[len] = 0;

	object->d_name = name;

	object->d_name_len = len;

	_leave(" = %s", object->d_name);

	return true;

}