linux-kernel - Re: [PATCH v12 19/28] riscv/ptrace: riscv cfi status and state via ptrace and in core files

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Message-Id: <D8LRI8WGXGVD.3B0VB9PCQ9I2R@ventanamicro.com>
Date: Fri, 21 Mar 2025 08:22:56 +0100
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Deepak Gupta" <debug@...osinc.com>
Cc: "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar"
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Subject: Re: [PATCH v12 19/28] riscv/ptrace: riscv cfi status and state via
 ptrace and in core files

2025-03-20T16:09:12-07:00, Deepak Gupta <debug@...osinc.com>:
> On Thu, Mar 20, 2025 at 3:24 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
>> 2025-03-14T14:39:38-07:00, Deepak Gupta <debug@...osinc.com>:
>> > Expose a new register type NT_RISCV_USER_CFI for risc-v cfi status and
>> > state. Intentionally both landing pad and shadow stack status and state
>> > are rolled into cfi state. Creating two different NT_RISCV_USER_XXX would
>> > not be useful and wastage of a note type. Enabling or disabling of feature
>> > is not allowed via ptrace set interface. However setting `elp` state or
>> > setting shadow stack pointer are allowed via ptrace set interface. It is
>> > expected `gdb` might have use to fixup `elp` state or `shadow stack`
>> > pointer.
>> >
>> > Signed-off-by: Deepak Gupta <debug@...osinc.com>
>> > ---
>> > diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
>> > index 659ea3af5680..e6571fba8a8a 100644
>> > @@ -131,6 +131,24 @@ struct __sc_riscv_cfi_state {
>> >       unsigned long ss_ptr;   /* shadow stack pointer */
>> >  };
>> >
>> > +struct __cfi_status {
>> > +     /* indirect branch tracking state */
>> > +     __u64 lp_en : 1;
>> > +     __u64 lp_lock : 1;
>> > +     __u64 elp_state : 1;
>> > +
>> > +     /* shadow stack status */
>> > +     __u64 shstk_en : 1;
>> > +     __u64 shstk_lock : 1;
>>
>> I remember there was deep hatred towards bitfields in the Linux
>> community, have things changes?
>
> hmm. I didn't know about the strong hatred.

There is a good reason for it. :)

The C standard left important behavior as implementation-specific (by
mistake, I hope).  I do like bitfields, but you have to be extra careful
when working with them.

> Although I can see lots of examples of this pattern in existing kernel code.
> No strong feelings on my side, I can change this and have it single 64bit field
> and accessed via bitmasks.

This is uapi and bitfields do not specify the internal representation.
A program compiled at a different time can see completely different
order of the bitfields, so the uapi would break.

We cannot use bitfields here.