Swatantra-66 opened a new pull request, #20981:
URL: https://github.com/apache/nuttx/pull/20981
## Summary
Fixes #18558.
The ARM Architecture Procedure Call Standard (AAPCS-32, ยง5.2.1.1) mandates:
- **SP mod 4 = 0**: The stack pointer must be word-aligned at all times.
- **SP mod 8 = 0**: The stack pointer must be **double-word (8-byte)
aligned** at each external interface (function entry and call boundary).
In NuttX on ARM architectures (`armv6-m`, `armv7-m`, `armv8-m`, `armv7-a`,
`armv7-r`, `armv8-r`), the idle thread stack pointer is defined relative to the
end of the BSS section:
```c
#define IDLE_STACK (_ebss + CONFIG_IDLETHREAD_STACKSIZE)
```
On Cortex-M cores, `IDLE_STACK` is placed at vector table index 0 and loaded
directly into the Main Stack Pointer (`MSP`) by hardware on reset. On Cortex-A
and Cortex-R cores, `SP` is loaded with `IDLE_STACK` in startup assembly
(`arm_head.S`).
Prior to this change, board linker scripts terminated `.bss` with:
```ld
*(COMMON)
. = ALIGN(4);
_ebss = ABSOLUTE(.);
} > sram
```
If `.bss` ended on a 4-byte boundary that was not a multiple of 8, `_ebss`
and the initial idle stack pointer `IDLE_STACK` were only 4-byte aligned (`SP %
8 == 4`).
When the idle thread invokes functions with 64-bit varargs (such as
`syslog()` with `CONFIG_SYSLOG_TIMESTAMP` enabled or formatting 64-bit integers
with `vsprintf_internal`), GCC's `va_arg` forcibly 8-byte aligns the `ap`
pointer (`bic.w r9, r9, #7`). On a 4-byte aligned stack, this advances `ap` by
12 bytes instead of 8 bytes, corrupting the varargs stream and causing
subsequent parameter dereferences to fail with Data Abort / Bus Fault
exceptions (as documented in issue #18558).
Commit 8409599ae447 (PR #18569) addressed this specifically for
`boards/arm/stm32*/`. This commit extends the fix across all remaining ARM32
boards (and any missed/newly added STM32 scripts) by aligning `_ebss` to an
8-byte boundary (`. = ALIGN(8);`).
## Impact
- **Architecture**: ARM32 (`armv6-m`, `armv7-m`, `armv8-m`, `armv7-a`,
`armv7-r`, `armv8-r`).
- **Functionality**: Guarantees that the idle task stack is always strictly
8-byte aligned at reset and throughout execution, conforming to AAPCS-32 and
preventing bus faults when 64-bit varargs / syslog timestamps are accessed in
the idle thread.
- **Compatibility**: Safe drop-in alignment enhancement; aligns with
existing STM32 implementation in commit 8409599ae447.
## Testing
- **Repository Audit**: Performed an automated audit of all 558 linker
scripts in `boards/arm/` to confirm zero remaining 4-byte aligned or unaligned
`_ebss` definitions.
- **Diff Check**: Verified clean `git diff --check` across all 341 modified
linker scripts (0 whitespace / newline errors).
- **Prior Art**: Aligns directly with merged upstream commit 8409599ae447
(`boards/arm/stm32*/: Align ARM STM32 idle stack`).
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