Re: [DTrace-devel] [PATCH 3/3] cg: fix masking of lower 32 bits

On 9/23/25 17:05, Kris Van Hees via DTrace-devel wrote:

> The "and %r0, 0xffffffff' instruction does not actually mask off the
> lower 32 bits as one would expect because 0xffffffff is interpreted as
> -1 and then sign-extedned to a 64-bit value, i.e. keeping all bits.

s/extedned/extended/

> The "mov32 %r0, %r0" instruxtion does correctly mask off the lwoer 32

s/instruxtion/instruction/
s/lwoer/lower/

I started looking at this a while back and was not convinced that the 
lower 32 bits were what we want.  Do we want them, and do we have a test 
that confirms?  Or do we want the upper 32 bits?  I got distracted and 
mothballed the whole thing, but the test involved spawning a child 
thread and then looking at its ustack()...  maybe after the child had 
exited but before the parent did.  Admittedly, I was confused at the 
time, and I'm even worse now.

> bits because it forced the value in %r0 to be a 32-bit value.
>
> Signed-off-by: Kris Van Hees <[email protected]>
> ---
>   include/bpf_asm.h | 11 +++++++++++
>   libdtrace/dt_cg.c |  3 ++-
>   2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/bpf_asm.h b/include/bpf_asm.h
> index 152d2312..63987e01 100644
> --- a/include/bpf_asm.h
> +++ b/include/bpf_asm.h
> @@ -36,6 +36,15 @@
>   		.imm = 0						\
>   	})
>   
> +#define BPF_ALU32_REG(op, dst, src)					\
> +	((struct bpf_insn) {						\
> +		.code = BPF_ALU | (op) | BPF_X,				\
> +		.dst_reg = (dst),					\
> +		.src_reg = (src),					\
> +		.off = 0,						\
> +		.imm = 0						\
> +	})
> +
>   #define BPF_END_REG(sz, dst, dir)					\
>   	((struct bpf_insn) {						\
>   		.code = BPF_ALU | BPF_END | (dir),			\
> @@ -68,6 +77,8 @@
>   #define BPF_MOV_REG(dst, src)	BPF_ALU64_REG(BPF_MOV, dst, src)
>   #define BPF_MOV_IMM(dst, val)	BPF_ALU64_IMM(BPF_MOV, dst, val)
>   
> +#define BPF_MOV32_REG(dst, src)	BPF_ALU32_REG(BPF_MOV, dst, src)
> +
>   #define BPF_LOAD(sz, dst, src, ofs)					\
>   	((struct bpf_insn) {						\
>   		.code = BPF_LDX | BPF_MEM | (sz),			\
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index a8f2c9d2..28b7e7c4 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -2757,7 +2757,8 @@ dt_cg_act_stack_sub(dt_pcb_t *pcb, dt_node_t *dnp, int reg, int off, dtrace_actk
>   		dt_regset_xalloc(drp, BPF_REG_0);
>   		emit(dlp,  BPF_CALL_HELPER(BPF_FUNC_get_current_pid_tgid));
>   		dt_regset_free_args(drp);
> -		emit(dlp,  BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xffffffff));
> +		/* mov32 %r0, %r0 effectively masks the lower 32 bits. */
> +		emit(dlp,  BPF_MOV32_REG(BPF_REG_0, BPF_REG_0));
>   		emit(dlp,  BPF_STORE(BPF_DW, reg, off, BPF_REG_0));
>   		dt_regset_free(drp, BPF_REG_0);
>   	}