[DTrace-devel] [PATCH] Fix DT_TOK_INT for values between INT32_MAX and UINT32_MAX

[Kris Van Hees]

On Wed, Jul 15, 2020 at 08:28:58PM -0400, eugene.loh at oracle.com wrote:
> From: Eugene Loh <eugene.loh at oracle.com>
> 
> An earlier commit
> ("Use MOV instead of LDDW when value is less than UINT32_MAX")
> attempted to optimize away one BPF instruction for storing a constant
> in a BPF register.  In the dt_cg_node() handling of DT_TOK_INT, a
> 64-bit dnp->dn_value constant must be stored in a 64-bit BPF register.
> In general, this uses an LDDW double instruction (via dt_cg_setx).
> If the value fits into the 32-bit IMM, however, one might get by
> with a single MOV_IMM instruction.  The earlier commit tested for
> this optimization opportunity by checking the value against UINT32_MAX
> -- that is, whether the value fits into 32 bits.
> 
> The problem with this approach is that if the uppermost bit of the IMM
> is set, then MOV_IMM will sign extend the value when it fills the BPF
> register.  For example,
> 
>         # dtrace -qn 'BEGIN {x = 3000000000; trace(x); exit(0)}'
>         -1294967296
> 
> Change the check to INT32_MAX.

Nice catch.

> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>

Reviewed-by; Kris Van Hees <kris.van.hees at oracle.com>

> ---
>  libdtrace/dt_cg.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index 26172432..b80e464d 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -2884,7 +2884,7 @@ if ((idp = dnp->dn_ident)->di_kind != DT_IDENT_FUNC)
>  		if ((dnp->dn_reg = dt_regset_alloc(drp)) == -1)
>  			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
>  
> -		if (dnp->dn_value > UINT32_MAX)
> +		if (dnp->dn_value > INT32_MAX)
>  			dt_cg_setx(dlp, dnp->dn_reg, dnp->dn_value);
>  		else {
>  			instr = BPF_MOV_IMM(dnp->dn_reg, dnp->dn_value);
> -- 
> 2.18.2
> 
> 
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