On 1/25/24 5:12 PM, dthaler1968@xxxxxxxxxxxxxx wrote:
The spec defines:
As discussed below in `64-bit immediate instructions`_, a 64-bit immediate
instruction uses a 64-bit immediate value that is constructed as follows.
The 64 bits following the basic instruction contain a pseudo instruction
using the same format but with opcode, dst_reg, src_reg, and offset all set to zero,
and imm containing the high 32 bits of the immediate value.
[...]
imm64 = (next_imm << 32) | imm
The 64-bit immediate instructions section then says:
Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction
encoding defined in `Instruction encoding`_, and use the 'src' field of the
basic instruction to hold an opcode subtype.
Some instructions then nicely state how to use the full 64 bit immediate value, such as
BPF_IMM | BPF_DW | BPF_LD 0x18 0x0 dst = imm64 integer integer
BPF_IMM | BPF_DW | BPF_LD 0x18 0x2 dst = map_val(map_by_fd(imm)) + next_imm map fd data pointer
BPF_IMM | BPF_DW | BPF_LD 0x18 0x6 dst = map_val(map_by_idx(imm)) + next_imm map index data pointer
Others don't:
BPF_IMM | BPF_DW | BPF_LD 0x18 0x1 dst = map_by_fd(imm) map fd map
BPF_IMM | BPF_DW | BPF_LD 0x18 0x3 dst = var_addr(imm) variable id data pointer
BPF_IMM | BPF_DW | BPF_LD 0x18 0x4 dst = code_addr(imm) integer code pointer
BPF_IMM | BPF_DW | BPF_LD 0x18 0x5 dst = map_by_idx(imm) map index map
How is next_imm used in those four? Must it be 0? Or can it be anything and it's ignored?
Or is it used for something?
The other four must have next_imm to be 0. No use of next_imm in thee four insns kindly implies this.
See uapi bpf.h for details (search BPF_PSEUDO_MAP_FD).
Dave
