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Thus, map_by_idx(imm) uses the fd at that index in the array.”…””}”(hj#h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´Khj÷h²hubeh}”(h]”j†ah ]”h"]”Œmaps”ah$]”h&]”uh1j hjh²hh³hËh´Kubj)”}”(hhh]”(h¼)”}”(hŒ Variables”h]”(há)”}”(hhh]”hŒ 1.4   ”…””}”(hj?h²hh³Nh´Nubah}”(h]”h ]”hìah"]”h$]”h&]”uh1hàhj;h²hh³Nh´NubhŒ Variables”…””}”(hj;h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”j-Kj.j¯uh1h»hj8h²hh³hËh´K ubh×)”}”(hŒœThe following 64-bit immediate instruction specifies that a variable address, which corresponds to some integer stored in the 'imm' field, should be loaded:”h]”hŒ The following 64-bit immediate instruction specifies that a variable address, which corresponds to some integer stored in the ‘imm’ field, should be loaded:”…””}”(hjVh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K"hj8h²hubhŒtable”“”)”}”(hhh]”hŒtgroup”“”)”}”(hhh]”(hŒcolspec”“”)”}”(hhh]”h}”(h]”h ]”h"]”h$]”h&]”Œcolwidth”Kuh1jnhjkubjo)”}”(hhh]”h}”(h]”h ]”h"]”h$]”h&]”Œcolwidth”Kuh1jnhjkubjo)”}”(hhh]”h}”(h]”h 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pointer”h]”hŒ data pointer”…””}”(hjÌh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K(hjÉubah}”(h]”h ]”h"]”h$]”h&]”uh1j¶hjSubeh}”(h]”h ]”h"]”h$]”h&]”uh1j±hjPubah}”(h]”h ]”h"]”h$]”h&]”uh1jNhjkubeh}”(h]”h ]”h"]”h$]”h&]”Œcols”Kuh1jihjfubah}”(h]”h ]”h"]”h$]”h&]”uh1jdhj8h²hh³hËh´Nubh×)”}”(hŒ#On Linux, this integer is a BTF ID.”h]”hŒ#On Linux, this integer is a BTF ID.”…””}”(hjùh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K+hj8h²hubeh}”(h]”jµah ]”h"]”Œ variables”ah$]”h&]”uh1j hjh²hh³hËh´K ubj)”}”(hhh]”(h¼)”}”(hŒ%Legacy BPF Packet access instructions”h]”(há)”}”(hhh]”hŒ 1.5   ”…””}”(hjh²hh³Nh´Nubah}”(h]”h ]”hìah"]”h$]”h&]”uh1hàhjh²hh³Nh´NubhŒ%Legacy BPF Packet access instructions”…””}”(hjh²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”j-Kj.jÞuh1h»hjh²hh³hËh´K.ubh×)”}”(hX.As mentioned in the `ISA standard documentation `_, Linux has special eBPF instructions for access to packet data that have been carried over from classic BPF to retain the performance of legacy socket filters running in the eBPF interpreter.”h]”(hŒAs mentioned in the ”…””}”(hj,h²hh³Nh´NubhÜ)”}”(hŒZ`ISA standard documentation `_”h]”hŒISA standard documentation”…””}”(hj4h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œname”ŒISA standard documentation”Œrefuri”Œ:instruction-set.html#legacy-bpf-packet-access-instructions”uh1hÛhj,ubhŒtarget”“”)”}”(hŒ= ”h]”h}”(h]”Œisa-standard-documentation”ah ]”h"]”Œisa standard documentation”ah$]”h&]”Œrefuri”jEuh1jFŒ referenced”Khj,ubhŒÀ, Linux has special eBPF instructions for access to packet data that have been carried over from classic BPF to retain the performance of legacy socket filters running in the eBPF interpreter.”…””}”(hj,h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K0hjh²hubh×)”}”(hŒdThe instructions come in two forms: ``BPF_ABS | | BPF_LD`` and ``BPF_IND | | BPF_LD``.”h]”(hŒ$The instructions come in two forms: ”…””}”(hj`h²hh³Nh´Nubj`)”}”(hŒ``BPF_ABS | | BPF_LD``”h]”hŒBPF_ABS | | BPF_LD”…””}”(hjhh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j_hj`ubhŒ and ”…””}”(hj`h²hh³Nh´Nubj`)”}”(hŒ``BPF_IND | | BPF_LD``”h]”hŒBPF_IND | | BPF_LD”…””}”(hjzh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j_hj`ubhŒ.”…””}”(hj`h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K6hjh²hubh×)”}”(hXUThese instructions are used to access packet data and can only be used when the program context is a pointer to a networking packet. ``BPF_ABS`` accesses packet data at an absolute offset specified by the immediate data and ``BPF_IND`` access packet data at an offset that includes the value of a register in addition to the immediate data.”h]”(hŒ†These instructions are used to access packet data and can only be used when the program context is a pointer to a networking packet. ”…””}”(hj’h²hh³Nh´Nubj`)”}”(hŒ ``BPF_ABS``”h]”hŒBPF_ABS”…””}”(hjšh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j_hj’ubhŒP accesses packet data at an absolute offset specified by the immediate data and ”…””}”(hj’h²hh³Nh´Nubj`)”}”(hŒ ``BPF_IND``”h]”hŒBPF_IND”…””}”(hj¬h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j_hj’ubhŒi access packet data at an offset that includes the value of a register in addition to the immediate data.”…””}”(hj’h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K9hjh²hubh×)”}”(hŒ0These instructions have seven implicit operands:”h]”hŒ0These instructions have seven implicit operands:”…””}”(hjÄh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´K?hjh²hubhÍ)”}”(hhh]”(hÒ)”}”(hŒQRegister R6 is an implicit input that must contain a pointer to a struct sk_buff.”h]”h×)”}”(hŒQRegister R6 is an implicit input that must contain a pointer to a struct sk_buff.”h]”hŒQRegister R6 is an implicit input that must contain a pointer to a struct sk_buff.”…””}”(hjÙh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´KAhjÕubah}”(h]”h ]”h"]”h$]”h&]”uh1hÑhjÒh²hh³hËh´NubhÒ)”}”(hŒRRegister R0 is an implicit output which contains the data fetched from the packet.”h]”h×)”}”(hŒRRegister R0 is an implicit output which contains the data fetched from the packet.”h]”hŒRRegister R0 is an implicit output which contains the data fetched from the packet.”…””}”(hjñh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´KChjíubah}”(h]”h ]”h"]”h$]”h&]”uh1hÑhjÒh²hh³hËh´NubhÒ)”}”(hŒMRegisters R1-R5 are scratch registers that are clobbered by the instruction. ”h]”h×)”}”(hŒLRegisters R1-R5 are scratch registers that are clobbered by the instruction.”h]”hŒLRegisters R1-R5 are scratch registers that are clobbered by the instruction.”…””}”(hj h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÖh³hËh´KEhjubah}”(h]”h ]”h"]”h$]”h&]”uh1hÑhjÒh²hh³hËh´Nubeh}”(h]”h ]”h"]”h$]”h&]”Œbullet”Œ*”uh1hÌh³hËh´KAhjh²hubh×)”}”(hŒ®These instructions have an implicit program exit condition as well. 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