Re: [DTrace-devel] [PATCH] Use DT_TRAMP_SP_SLOT() for BPF stack scratch space in trampoline

From: Kris Van Hees <kris.van.hees@oracle.com>
To: eugene.loh@oracle.com
Cc: dtrace@lists.linux.dev, dtrace-devel@oss.oracle.com
Subject: Re: [DTrace-devel] [PATCH] Use DT_TRAMP_SP_SLOT() for BPF stack scratch space in trampoline
Date: Fri, 21 Feb 2025 15:20:27 -0500	[thread overview]
Message-ID: <Z7jgC2xdXjT3L8Y3@oracle.com> (raw)
In-Reply-To: <20250221200924.4445-1-eugene.loh@oracle.com>

On Fri, Feb 21, 2025 at 03:09:24PM -0500, eugene.loh--- via DTrace-devel wrote:
> From: Eugene Loh <eugene.loh@oracle.com>
> 
> We might as well get this code right, even if this "fix" is
> arguably irrelevant for two reasons:
> 
> *)  The offset just so happens to be -96 before and after the
>     change anyhow, just by coincidence.
> 
> *)  The fix is on a code path that is not currently in use.
> 
> Signed-off-by: Eugene Loh <eugene.loh@oracle.com>

Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

> ---
>  libdtrace/dt_cg.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index 846f0fe2e..cc64a3a13 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -594,11 +594,11 @@ dt_cg_tramp_copy_pc_from_regs(dt_pcb_t *pcb)
>  		/* test just a single byte */
>  		emit(dlp,  BPF_MOV_IMM(BPF_REG_2, 1));
>  
> -		/* safe to write to FP+DT_STK_SP_BASE, which becomes the clause stack */
> +		/* write to scratch space */
>  		emit(dlp,  BPF_MOV_REG(BPF_REG_1, BPF_REG_FP));
> -		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, DT_STK_SP_BASE));
> +		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, DT_TRAMP_SP_SLOT(0)));
>  
> -		/* bpf_probe_read_kernel(%fp + DT_STK_SP, 1, PC) */
> +		/* bpf_probe_read_kernel(%fp + DT_TRAMP_SP_SLOT(0), 1, PC) */
>  		dt_regset_xalloc(drp, BPF_REG_0);
>  		emit(dlp,  BPF_CALL_HELPER(BPF_FUNC_probe_read_kernel));
>  
> -- 
> 2.43.5
> 
> 
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