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SubjectRe: [PATCH 1/2] riscv: ptrace: Use the correct API for `fcsr' access
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On Thu, 23 Jul 2020 16:22:15 PDT (-0700), macro@wdc.com wrote:
> Adjust the calls to `user_regset_copyout' and `user_regset_copyin' in
> `riscv_fpr_get' and `riscv_fpr_set' respectively so as to use @start_pos
> and @end_pos according to API documentation in <linux/regset.h>, that is
> to point at the beginning and the end respectively of the data chunk to
> be copied. Update @data accordingly, also for the first call, to make
> it clear which structure member is accessed.
>
> We currently have @start_pos fixed at 0 across all calls, which works as
> a result of the implementation, in particular because we have no padding
> between the FP general registers and the FP control and status register,
> but appears not to have been the intent of the API and is not what other
> ports do, requiring one to study the copy handlers to understand what is
> going on here.
>
> Signed-off-by: Maciej W. Rozycki <macro@wdc.com>
> Fixes: b8c8a9590e4f ("RISC-V: Add FP register ptrace support for gdb.")
> Cc: stable@vger.kernel.org # 4.20+
> ---
> arch/riscv/kernel/ptrace.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> linux-riscv-ptrace-fcsr.diff
> Index: linux-hv/arch/riscv/kernel/ptrace.c
> ===================================================================
> --- linux-hv.orig/arch/riscv/kernel/ptrace.c
> +++ linux-hv/arch/riscv/kernel/ptrace.c
> @@ -61,10 +61,13 @@ static int riscv_fpr_get(struct task_str
> int ret;
> struct __riscv_d_ext_state *fstate = &target->thread.fstate;
>
> - ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, fstate, 0,
> + ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &fstate->f, 0,
> offsetof(struct __riscv_d_ext_state, fcsr));

As far as I can tell the current code works correctly, it just requires
knowledge of the layout of __riscv_d_ext_state to determine that it functions
correctly. This new code still requires that knowledge: the first blob copies
the F registers, but only works if the CSR is after the registers. If we fix
both of those the code seems easier to read, but I don't think splitting the
difference helps any.

So I guess what I'm saying is: maybe that second line should be changed to
something like "ARRAY_SIZE(fstate->f)"?

> if (!ret) {
> - ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, fstate, 0,
> + ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf,
> + &fstate->fcsr,
> + offsetof(struct __riscv_d_ext_state,
> + fcsr),
> offsetof(struct __riscv_d_ext_state, fcsr) +
> sizeof(fstate->fcsr));
> }
> @@ -80,10 +83,13 @@ static int riscv_fpr_set(struct task_str
> int ret;
> struct __riscv_d_ext_state *fstate = &target->thread.fstate;
>
> - ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, fstate, 0,
> + ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &fstate->f, 0,
> offsetof(struct __riscv_d_ext_state, fcsr));
> if (!ret) {
> - ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, fstate, 0,
> + ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
> + &fstate->fcsr,
> + offsetof(struct __riscv_d_ext_state,
> + fcsr),
> offsetof(struct __riscv_d_ext_state, fcsr) +
> sizeof(fstate->fcsr));
> }

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