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Document that contexts must be movable

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Fix #4451.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2021-06-01 21:30:51 +02:00
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docs/architecture/alternative-implementations.md
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@ -51,6 +51,18 @@ Where a context type needs to have a certain field, the field must have the same
* ECP: `mbedtls_ecp_group` must have the fields `id`, `P`, `A`, `B`, `G`, `N`, `pbits` and `nbits`.
* If `MBEDTLS_PK_PARSE_EC_EXTENDED` is enabled, those fields must be writable, and `mbedtls_ecp_point_read_binary()` must support a group structure where only `P`, `pbits`, `A` and `B` are set.
It must be possible to move a context object in memory (except during the execution of a library function that takes this context as an argument). (This is necessary, for example, to support applications that populate a context on the stack of an inner function and then copy the context upwards through the call chain, or applications written in a language with automatic memory management that can move objects on the heap.) That is, call sequences like the following must work:
```
mbedtls_xxx_context ctx1, ctx2;
mbedtls_xxx_init(&ctx1);
mbedtls_xxx_setup(&ctx1, …);
ctx2 = ctx1;
memset(&ctx1, 0, sizeof(ctx1));
mbedtls_xxx_do_stuff(&ctx2, …);
mbedtls_xxx_free(&ctx2);
```
In practice, this means that a pointer to a context or to a part of a context does not remain valid across function calls. Alternative implementations do not need to support copying of contexts: contexts can only be cloned through explicit `clone()` functions.
## Function alternative implementations
In some cases, it is possible to replace a single function or a small set of functions instead of [providing an alternative implementation of the whole module](#module-alternative-implementations).