{bp-19123} armv7-m/armv8-m: honor SP modifications on signal return#19284
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On ARMv7-M/ARMv8-M, the hardware exception return determines the final SP from the physical location of the HW exception frame, ignoring any software modification to REG_R13 in the saved register context. This means signal handlers that adjust SP have their change silently discarded. Fix this by relocating the saved context in arm_sigdeliver before calling arm_fullcontextrestore. If the desired SP (regs[REG_R13]) differs from the implied SP, memmove the entire context frame so that the hardware exception return produces the correct final SP. The fix runs only in the signal return path, adding zero overhead to the normal exception return hot path. Signed-off-by: Andrew Au <cshung@gmail.com>
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linguini1
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Jul 3, 2026
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Summary
On ARMv7-M/ARMv8-M, the hardware exception return determines the final SP from the physical location of the HW exception frame, ignoring any software modification to REG_R13 in the saved register context. This means signal handlers that adjust SP have their change silently discarded.
Fix this by relocating the saved context in arm_sigdeliver before calling arm_fullcontextrestore. If the desired SP (regs[REG_R13]) differs from the implied SP, memmove the entire context frame so that the hardware exception return produces the correct final SP.
The fix runs only in the signal return path, adding zero overhead to the normal exception return hot path.
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