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doc: add data semantic example to author guide

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This commit is contained in:
Joe Lawrence 2017-03-24 16:43:52 -04:00
parent d072845816
commit de4042304c
1 changed files with 49 additions and 3 deletions
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52
doc/patch-author-guide.md
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@ -218,10 +218,56 @@ safe to access:
Data semantic changes
---------------------
Sometimes, the data itself remains the same, but how it's used is changed. A
common example is locking semantic changes.
Part of the stable-tree [backport](https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/fs/aio.c?h=linux-3.10.y&id=6745cb91b5ec93a1b34221279863926fba43d0d7)
to fix CVE-2014-0206 changed the reference count semantic of `struct
kioctx.reqs_active`. Associating a shadow variable to new instances of this
structure can be used by patched code to handle both new (post-patch) and
existing (pre-patch) instances.
Example needed.
(This example is trimmed to highlight this use-case. Boilerplate code is also
required to allocate/free a shadow variable called "reqs_active_v2" whenever a
new `struct kioctx` is created/released. No values are ever assigned to the
shadow variable.)
Shadow variable existence can be verified before applying the new data
semantic of the associated object:
```
@@ -678,6 +688,9 @@ void aio_complete(struct kiocb *iocb, lo
put_rq:
/* everything turned out well, dispose of the aiocb. */
aio_put_req(iocb);
+ reqs_active_v2 = kpatch_shadow_get(ctx, "reqs_active_v2");
+ if (reqs_active_v2)
+ atomic_dec(&ctx->reqs_active);
/*
* We have to order our ring_info tail store above and test
```
Likewise, shadow variable non-existence can be tested to continue applying the
old data semantic:
```
@@ -705,6 +718,7 @@ static long aio_read_events_ring(struct
unsigned head, pos;
long ret = 0;
int copy_ret;
+ int *reqs_active_v2;
mutex_lock(&ctx->ring_lock);
@@ -756,7 +770,9 @@ static long aio_read_events_ring(struct
pr_debug("%li h%u t%u\n", ret, head, ctx->tail);
- atomic_sub(ret, &ctx->reqs_active);
+ reqs_active_v2 = kpatch_shadow_get(ctx, "reqs_active_v2");
+ if (!reqs_active_v2)
+ atomic_sub(ret, &ctx->reqs_active);
out:
mutex_unlock(&ctx->ring_lock);
```
Locking semantic example needed.
Init code changes
-----------------