Re: [PATCH] Re-map value of NT_RISCV_CSR to not collide with the value of NT_RISCV_VECTOR in Linux kernel header file 'include/uapi/linux/elf.h'

John Baldwin
2023-08-10T18:39:51Z

On 8/9/23 10:53 AM, Greg Savin via Gdb-patches wrote:
> Linux kernel's file 'include/uapi/linux/elf.h' declares the value 0x900
> for NT_RISCV_VECTOR, and does not have a definition for NT_RISCV_CSR, nor
> does it use the value 0x901 for any note type. This patch is intended
> as a way to resolve the disagreement/collision between Linux and binutils,
> over the meaning of 0x900 in the context of note types.
>
> ---
> include/elf/common.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/elf/common.h b/include/elf/common.h
> index ffa6b60bd2b..0bbe245519e 100644
> --- a/include/elf/common.h
> +++ b/include/elf/common.h
> @@ -713,7 +713,7 @@
> /* note name must be "LINUX". */
> #define NT_LARCH_LBT 0xa04 /* LoongArch Binary Translation registers */
> /* note name must be "CORE". */
> -#define NT_RISCV_CSR 0x900 /* RISC-V Control and Status Registers */
> +#define NT_RISCV_CSR 0x901 /* RISC-V Control and Status Registers */
> /* note name must be "LINUX". */
> #define NT_SIGINFO 0x53494749 /* Fields of siginfo_t. */
> #define NT_FILE 0x46494c45 /* Description of mapped files. */

If there aren't any active uses of NT_RISCV_CSR "in the wild", then it might be
better to just remove it entirely?

It looks like commit e214f8db56f65531b0a5ec296c42339dcaa5af31 adding LoongArch
constants inadvertently changed the comment for this note from "CORE" (a
binutils note) to "LINUX" (a Linux kernel note).

The log message adding NT_RISCV_CSR is clearer (though the comment in common.h
is wrong and should be "GDB" it seems):

commit db6092f3aec43ea4d10efc5ff74274f04cdc0ad6
Author: Andrew Burgess <andrew.burgess@embecosm.com>
Date: Fri Nov 27 14:04:16 2020 +0000

bfd/binutils: add support for RISC-V CSRs in core files

Adds support for including RISC-V control and status registers into
core files.

The value for the define NT_RISCV_CSR is set to 0x900, this
corresponds to a patch I have proposed for the Linux kernel here:

http://lists.infradead.org/pipermail/linux-riscv/2020-December/003910.html

As I have not yet heard if the above patch will be accepted into the
kernel or not I have set the note name string to "GDB", and the note
type to NT_RISCV_CSR.

This means that if the above patch is rejected from the kernel, and
the note type number 0x900 is assigned to some other note type, we
will still be able to distinguish between the GDB produced
NT_RISCV_CSR, and the kernel produced notes, where the name would be
set to "CORE".

Seems like the kernel patch died on the vine (no responses to the patch).
GDB does support writing this out still though via the ".reg.riscv-csr"
pseudo section, so changing this would break debugging bare metal core
dumps. You can however, just re-use 0x900 for NT_RISCV_VECTOR without
this patch. The different note name is sufficient to differentiate the
two.

--
John Baldwin