[PATCH v4] riscv: hwprobe: Fix stale vDSO data for late-initialized keys at boot

[Olof Johansson]

Hi,

On Sat, Jun 28, 2025 at 01:27:42AM +0800, Jingwei Wang wrote:
> The value for some hwprobe keys, like MISALIGNED_VECTOR_PERF, is
> determined by an asynchronous kthread. This kthread can finish after
> the hwprobe vDSO data is populated, creating a race condition where
> userspace can read stale values.
> 
> A completion-based framework is introduced to synchronize the async
> probes with the vDSO population. The init_hwprobe_vdso_data()
> function is deferred to `late_initcall` and now blocks until all
> probes signal completion.
> 
> Reported-by: Tsukasa OI <research_trasio at irq.a4lg.com>
> Closes: https://lore.kernel.org/linux-riscv/760d637b-b13b-4518-b6bf-883d55d44e7f@irq.a4lg.com/
> Fixes: e7c9d66e313b ("RISC-V: Report vector unaligned access speed hwprobe")
> Cc: Palmer Dabbelt <palmer at dabbelt.com>
> Cc: Alexandre Ghiti <alexghiti at rivosinc.com>
> Cc: stable at vger.kernel.org
> Signed-off-by: Jingwei Wang <wangjingwei at iscas.ac.cn>
> ---
> Changes in v4:
> 	- Reworked the synchronization mechanism based on feedback from Palmer
>     	and Alexandre.
> 	- Instead of a post-hoc refresh, this version introduces a robust
> 	completion-based framework using an atomic counter to ensure async
> 	probes are finished before populating the vDSO.
> 	- Moved the vdso data initialization to a late_initcall to avoid
> 	impacting boot time.
> 
> Changes in v3:
> 	- Retained existing blank line.
> 
> Changes in v2:
> 	- Addressed feedback from Yixun's regarding #ifdef CONFIG_MMU usage.
> 	- Updated commit message to provide a high-level summary.
> 	- Added Fixes tag for commit e7c9d66e313b.
> 
> v1: https://lore.kernel.org/linux-riscv/20250521052754.185231-1-wangjingwei@iscas.ac.cn/T/#u
> 
>  arch/riscv/include/asm/hwprobe.h           |  8 +++++++-
>  arch/riscv/kernel/sys_hwprobe.c            | 20 +++++++++++++++++++-
>  arch/riscv/kernel/unaligned_access_speed.c |  9 +++++++--
>  3 files changed, 33 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 7fe0a379474ae2c6..87af186d92e75ddb 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -40,5 +40,11 @@ static inline bool riscv_hwprobe_pair_cmp(struct riscv_hwprobe *pair,
>  
>  	return pair->value == other_pair->value;
>  }
> -
> +#ifdef CONFIG_MMU
> +void riscv_hwprobe_register_async_probe(void);
> +void riscv_hwprobe_complete_async_probe(void);
> +#else
> +inline void riscv_hwprobe_register_async_probe(void) {}
> +inline void riscv_hwprobe_complete_async_probe(void) {}

These need to be:

static inline void riscv_hwprobe_register_async_probe(void) {}
static inline void riscv_hwprobe_complete_async_probe(void) {}

Or else you get an global instantiation of them in every file that includes
them, and compilation errors about duplicate symbols, as seen by
nommu_virt_defconfig:

riscv64-linux-gnu-ld: arch/riscv/kernel/process.o: in function `riscv_hwprobe_register_async_probe':
process.c:(.text+0x170): multiple definition of `riscv_hwprobe_register_async_probe'; arch/riscv/kernel/cpufeature.o:cpufeature.c:(.text+0x312): first defined here
riscv64-linux-gnu-ld: arch/riscv/kernel/process.o: in function `riscv_hwprobe_complete_async_probe':
process.c:(.text+0x17c): multiple definition of `riscv_hwprobe_complete_async_probe'; arch/riscv/kernel/cpufeature.o:cpufeature.c:(.text+0x31e): first defined here
riscv64-linux-gnu-ld: arch/riscv/kernel/ptrace.o: in function `riscv_hwprobe_register_async_probe':
ptrace.c:(.text+0x714): multiple definition of `riscv_hwprobe_register_async_probe'; arch/riscv/kernel/cpufeature.o:cpufeature.c:(.text+0x312): first defined here



-Olof