RIOT-OS/RIOT
RIOT-OS/RIOT
1
0
Fork 0
mirror of https://github.com/RIOT-OS/RIOT.git synced 2024-12-29 04:50:03 +01:00
Code Releases Activity

Merge #19619

Browse Source 
19619: cpu/arm7: Fix undefined behavior based on invalid assembly r=aabadie a=Teufelchen1

### Contribution description

Hellooo! 🦤

In this PR a single `NOP` is added after an `ldm` in ARM7 common code in the scheduling management.
The change is necessary because this particular `ldm` affects certain banked registers, including `R14 / lr`. The next instruction then immediately accesses `lr`. This is invalid and the exact effect depends on the underlying hardware implementation. An intermediate `NOP` ensures correct behaviour in such cases.

The ARM docs can be found [here](https://documentation-service.arm.com/static/5f8dacc8f86e16515cdb865a), in section `A4.1.21 LDM (2)`. It states:
> In ARM architecture versions earlier than ARMv6, this form of LDM must not be
followed by an instruction that accesses banked registers. A following NOP is a good
way to ensure this.


### Testing procedure

Without this change, on the Game Boy Advance, RIOT behaves irregularly. With the `NOP`, it works fine!
If possible, this change should be tested on other ARM7 hardware as well but I do not have access to any.  

----

Special thanks `@pyropeter` for spotting the extremely tiny note on this issue within the ARM docs. You saved me hours of tears! 👾

Co-authored-by: Teufelchen1 <bennet.blischke@outlook.com>
This commit is contained in:
bors[bot] 2023-05-20 12:51:14 +00:00 committed by GitHub
commit 06e27d5aba
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 4 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
cpu/arm7_common/common.s
View File

@ -101,6 +101,10 @@ task_return:
/* Restore all registers, PC & cpsr */
ldmfd r0, {r0-lr}^
/* According to ARM Architecture Reference Manual DDI 0100I,
* ldms which affect banked registers must be followed by a nop.
*/
nop
/* return to task */
movs pc, lr