Re: [PATCH] riscv: kgdb: Ensure that BUFMAX > NUMREGBYTES

["Miquel Sabaté Solà" <mikisabate@gmail.com>]

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Miquel Sabaté Solà @ 2025-09-15 16:32 +02:

> The current value of BUFMAX is similar as in other architectures, but as
> per documentation on KGDB (see
> 'Documentation/process/debugging/kgdb.rst'), BUFMAX has to be larger
> than NUMREGBYTES.
>
> Some NUMREGBYTES architectures (e.g. powerpc or hexagon) actually define
> BUFMAX in relation to NUMREGBYTES, and thus this condition is always
> guaranteed. Since 2048 is a value that is generally accepted on all
> architectures, and that is larger than the current value of NUMREGBYTES,
> we can keep this value in arch/riscv, but we can at least add an
> 'static_assert' as an extra measure just in case NUMREGBYTES changes in
> the future for some unforseen reason.
>
> Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
> ---
>  arch/riscv/include/asm/kgdb.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/kgdb.h b/arch/riscv/include/asm/kgdb.h
> index cc11c4544cff..66ff46e434fa 100644
> --- a/arch/riscv/include/asm/kgdb.h
> +++ b/arch/riscv/include/asm/kgdb.h
> @@ -3,14 +3,18 @@
>  #ifndef __ASM_KGDB_H_
>  #define __ASM_KGDB_H_
>
> +#include <linux/build_bug.h>
> +
>  #ifdef __KERNEL__
>
>  #define GDB_SIZEOF_REG sizeof(unsigned long)
>
> -#define DBG_MAX_REG_NUM (36)
> -#define NUMREGBYTES ((DBG_MAX_REG_NUM) * GDB_SIZEOF_REG)
> +#define DBG_MAX_REG_NUM 36
> +#define NUMREGBYTES (DBG_MAX_REG_NUM * GDB_SIZEOF_REG)
>  #define CACHE_FLUSH_IS_SAFE     1
>  #define BUFMAX                  2048
> +static_assert(BUFMAX > NUMREGBYTES,
> +	      "As per KGDB documentation, BUFMAX must be larger than NUMREGBYTES");
>  #ifdef CONFIG_RISCV_ISA_C
>  #define BREAK_INSTR_SIZE	2
>  #else
> @@ -97,6 +101,7 @@ extern unsigned long kgdb_compiled_break;
>  #define DBG_REG_STATUS_OFF 33
>  #define DBG_REG_BADADDR_OFF 34
>  #define DBG_REG_CAUSE_OFF 35
> +/* NOTE: increase DBG_MAX_REG_NUM if you add more values here. */
>
>  extern const char riscv_gdb_stub_feature[64];

Gentle ping :)

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