kernel: force K_FP_REGS in z_setup_new_thread when FPU_SHARING - #10
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When CONFIG_FPU and CONFIG_FPU_SHARING are both enabled, GCC >= 9 with -mfloat-abi=hard emits FP instructions in any translation unit for struct copies and register spills, regardless of whether the source code explicitly uses float. This affects all threads, including kernel-internal ones like main and sysworkq whose options are set by the kernel, not the application. On Cortex-M, arch_new_thread() reads K_FP_REGS at creation time to size the MPU stack guard (FP_GUARD_EXTRA_SIZE) and set Z_ARM_MODE_MPU_GUARD_FLOAT_Msk. Without K_FP_REGS at that point, the guard is sized too small. z_arm_mpu_stack_guard_and_fpu_adjust() can retroactively tag K_FP_REGS at the next context switch, but any ISR arriving between the thread's first FP instruction and that switch opens the Cortex-M4 ES0182 lazy-FPU-stacking race window, corrupting the saved PC and causing a compound HARD+USAGE+BUS+MPU fault on exception return. Fix this by unconditionally ORing K_FP_REGS into options at the top of z_setup_new_thread() when CONFIG_FPU && CONFIG_FPU_SHARING, before z_init_thread_base() writes user_options and before arch_new_thread() reads them for guard sizing. This closes the race for main, sysworkq, idle, and any future kernel-internal thread in one place without requiring each call-site to be updated. Fixes zephyrproject-rtos#108793 Signed-off-by: BhuvanChandra DV <[email protected]>
Gowtham1256
approved these changes
Jul 3, 2026
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Summary
Fixes the ES0182 lazy-FPU-stacking race window on Cortex-M for all kernel-internal threads (main, sysworkq, idle) when
CONFIG_FPU && CONFIG_FPU_SHARINGare enabled.-mfloat-abi=hardemits FP instructions in any thread for struct copies and register spills, even without explicitfloatin sourcearch_new_thread()readsK_FP_REGSat creation time to size the MPU stack guard — if absent, the guard is sized too smallz_arm_mpu_stack_guard_and_fpu_adjust()can only retroactively tagK_FP_REGSat the next context switch, leaving a race window open for any ISR that fires before that switchK_FP_REGSintooptionsat the top ofz_setup_new_thread(), beforez_init_thread_base()andarch_new_thread()— closes the gap for all threads in one placeUpstream issue: zephyrproject-rtos#108793
Changes
kernel/thread.c: addoptions |= K_FP_REGSunder#if defined(CONFIG_FPU) && defined(CONFIG_FPU_SHARING)at the top ofz_setup_new_thread()Test plan
oksbot_base_ctrl_fwwithCONFIG_FPU=y,CONFIG_FPU_SHARING=y,CONFIG_HW_STACK_PROTECTION=y— verify no boot crash duringboot_banner()/ static C++ constructorsK_FP_REGSset via GDBk_thread_foreachafter boot