content/browser/cache_storage/README.md - Issue 2901083002: [CacheStorage] Pad and bin opaque resource sizes.

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Issue 2901083002: [CacheStorage] Pad and bin opaque resource sizes. (Closed)

Patch Set: Patch Set 8 changes. Created 3 years, 4 months ago

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1 # Architecture (as of July 29th 2016)	1 # Architecture (as of July 29th 2016)

2 This document descibes the browser-process implementation of the [Cache	2 This document describes the browser-process implementation of the [Cache

3 Storage specification](	3 Storage specification](

4 https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html).	4 https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html).

5	5

6 ## Major Classes and Ownership	6 ## Major Classes and Ownership

7 ### Ownership	7 ### Ownership

8 Where '=>' represents ownership, '->' is a reference, and '~>' is a weak	8 Where '=>' represents ownership, '->' is a reference, and '~>' is a weak

9 reference.	9 reference.

10	10

11 ##### `CacheStorageContextImpl`=>`CacheStorageManager`=>`CacheStorage`=>`CacheSt orageCache`	11 ##### `CacheStorageContextImpl`=>`CacheStorageManager`=>`CacheStorage`=>`CacheSt orageCache`

12 * A `CacheStorageManager` can own multiple `CacheStorage` objects.	12 * A `CacheStorageManager` can own multiple `CacheStorage` objects.

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145 `CacheStorage` as well. This has happened in the past (`Cache::Put` called	145 `CacheStorage` as well. This has happened in the past (`Cache::Put` called

146 `QuotaManager` to determine how much room was available, which in turn called	146 `QuotaManager` to determine how much room was available, which in turn called

147 `Cache::Size`). Be careful to avoid situations in which one operation triggers	147 `Cache::Size`). Be careful to avoid situations in which one operation triggers

148 a dependency on another operation from the same scheduler.	148 a dependency on another operation from the same scheduler.

149	149

150 At the end of an operation, the scheduler needs to be kicked to start the next	150 At the end of an operation, the scheduler needs to be kicked to start the next

151 operation. The idiom for this in CacheStorage/ is to wrap the operation's	151 operation. The idiom for this in CacheStorage/ is to wrap the operation's

152 callback with a function that will run the callback as well as advance the	152 callback with a function that will run the callback as well as advance the

153 scheduler. So long as the operation runs its wrapped callback the scheduler	153 scheduler. So long as the operation runs its wrapped callback the scheduler

154 will advance.	154 will advance.

	155

	156 ## Opaque Resource Size Obfuscation

	157 Applications can cache cross-origin resources as per

	158 [Cross-Origin Resources and CORS](https://www.w3.org/TR/service-workers-1/#cross -origin-resources).

	159 Opaque responses may only be cached, and in order to prevent "leaking" the size
	jkarlin 2017/07/31 14:03:24 what do you mean "may only be cached". What is it what do you mean "may only be cached". What is it that they can't do? cmumford 2017/08/10 16:37:10 haha - yeah, good point. Show quoted text On 2017/07/31 14:03:24, jkarlin wrote: > what do you mean "may only be cached". What is it that they can't do? haha - yeah, good point.
	160 of opaque responses their sizes are obfuscated. Random padding is added to the

	161 actual size making it difficult for an attacker to ascertain the actual resource

	162 size.
	jkarlin 2017/07/31 14:03:24 actual resource size via quota apis. actual resource size via quota apis. cmumford 2017/08/10 16:37:10 Done. Show quoted text On 2017/07/31 14:03:24, jkarlin wrote: > actual resource size via quota apis. Done.
	163

	164 When Chromium starts, a new random padding key is generated and used

	165 for all new caches created. This key is used by each cache to calculate padding

	166 for opaque resources. This key is also persisted to the cache index.
	jkarlin 2017/07/31 14:03:24 How about replacing the last line with: Each cach How about replacing the last line with: Each cache's key is persisted to disk in the cache index file. cmumford 2017/08/10 16:37:10 Done. Show quoted text On 2017/07/31 14:03:24, jkarlin wrote: > How about replacing the last line with: > > Each cache's key is persisted to disk in the cache index file. Done.
	167

	168 Each cache maintains the total padding for all opaque resources within the

	169 cache. This padding is added to the actual resource size when reporting sizes

	170 to the quota manager.

	171

	172 The algorithm version is also written to each cache. This allows for the
	jkarlin 2017/07/31 14:03:24 s/algorithm/padding algorithm/ s/algorithm/padding algorithm/ cmumford 2017/08/10 16:37:10 Done. Show quoted text On 2017/07/31 14:03:24, jkarlin wrote: > s/algorithm/padding algorithm/ Done.
	173 algorithm to be changed at a future date. CacheStorage will use the persisted

	174 key and padding from the cache's index unless the algorithm has been changed,

	175 one of values is missing, or deemed to be incorrect. In this situation the cache

	176 is enumerated and the padding recalculated during open.

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