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X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTIwMDAyOSBTYWx0ZWRfX3RXsNLgJA2Le KLmldr3+gPo6Ya6pm+W7neFNMgmS+Y870j6nTTrOdvkE9lLcxtMnHm61shCUA8F4sMdxIBqzfTt 1lA1zmWd/Ua4fVnORWmLp+80PiI0ixfNNtE/bG7bI5X913bIv8BlHfdR9I+vMWb/SqRh8DHryKi Z7y9GVx+ioVf4rBOQbz9UHsr2Ygb+9yiQbx/udrLXQB8fG/iM3D101FuoWMQJy1b/dCsV3PVjed zGKqP8Ne4OG/3fKRsJcDgccwc8uF6eekymsgpPZ0fvMGcz9jZ/YTyYvb6Kmaa0R4BDpIedmdC4X BSmimwKVRHaLX4RAO3Uxi9PdMcaMF2BPHMFqVcWnVG8MdPAV/Ruq5g7Bh/ziwIPqEf7xj5sRrg0 wsIeqZ24 X-Authority-Analysis: v=2.4 cv=Vfv3PEp9 c=1 sm=1 tr=0 ts=68d31f78 cx=c_pps a=OOZaFjgC48PWsiFpTAqLcw==:117 a=OOZaFjgC48PWsiFpTAqLcw==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=yJojWOMRYYMA:10 a=GoEa3M9JfhUA:10 a=yPCof4ZbAAAA:8 a=gJAts9LzvbAACuw7y8gA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 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 > --- > 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); > }