Add eBPF instruction CALLX for indirect calls

When clang encounters indirect calls in eBPF programs, it emits a call
instruction with a register parameter (`BPF_X`) instead of an immediate
value (`BPF_K`). This encoding (`BPF_JMP | BPF_CALL | BPF_X = 0x8d`) is
decoded by llvm-objdump as `callx`.

For example, here is a simple C program with an indirect call:

    extern void (*ptr_to_some_function)(void);
    void call_ptr_to_some_function(void) {
        ptr_to_some_function();
    }

Compiling and disassembling it gives with clang 14.0 (and LLVM 14.0):

    $ clang -O2 -target bpf -c indirect_call.c -o indirect_call.ebpf
    $ llvm-objdump -rd indirect_call.ebpf

    indirect_call.ebpf:  file format elf64-bpf

    Disassembly of section .text:

    0000000000000000 <call_ptr_to_some_function>:
           0:  18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00  r1 = 0 ll
                    0000000000000000:  R_BPF_64_64  ptr_to_some_function
           2:  79 11 00 00 00 00 00 00  r1 = *(u64 *)(r1 + 0)
           3:  8d 00 00 00 01 00 00 00  callx r1
           4:  95 00 00 00 00 00 00 00  exit

Contrary to usual eBPF instruction, `callx`'s register operand is
encoded in the immediate field. This encoding is actually specific to
LLVM (and clang). GCC used the destination register to store the target
register.

LLVM 19.1 was modified to use GCC's encoding:
llvm/llvm-project#81546 ("BPF: Change callx insn
encoding"). For example, in an Alpine Linux 3.21 system:

    $ clang -target bpf --version
    Alpine clang version 19.1.4
    Target: bpf
    Thread model: posix
    InstalledDir: /usr/lib/llvm19/bin

    $ clang -O2 -target bpf -c indirect_call.c -o indirect_call.ebpf
    $ llvm-objdump -rd indirect_call.ebpf

    indirect_call.ebpf:  file format elf64-bpf

    Disassembly of section .text:

    0000000000000000 <call_ptr_to_some_function>:
           0:  18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00  r1 = 0x0 ll
                    0000000000000000:  R_BPF_64_64  ptr_to_some_function
           2:  79 11 00 00 00 00 00 00  r1 = *(u64 *)(r1 + 0x0)
           3:  8d 01 00 00 00 00 00 00  callx r1
           4:  95 00 00 00 00 00 00 00  exit

The instruction is now encoded `8d 01 00...`.

For reference, here are similar commands using GCC showing it is using
the same encoding (here, compiler option `-mxbpf` is required to enable
several features including indirect calls, cf.
https://gcc.gnu.org/onlinedocs/gcc-12.4.0/gcc/eBPF-Options.html ).

    $ bpf-gcc --version
    bpf-gcc (12-20220319-1ubuntu1+2) 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f]

    $ bpf-gcc -O2 -c indirect_call.c -o indirect_call.ebpf -mxbpf
    $ bpf-objdump -mxbpf -rd indirect_call.ebpf

    indirect_call_gcc-12.ebpf:     file format elf64-bpfle

    Disassembly of section .text:

    0000000000000000 <call_ptr_to_some_function>:
       0:  18 00 00 00 00 00 00 00   lddw %r0,0
       8:  00 00 00 00 00 00 00 00
          0: R_BPF_INSN_64  ptr_to_some_function
      10:  79 01 00 00 00 00 00 00   ldxdw %r1,[%r0+0]
      18:  8d 01 00 00 00 00 00 00   call %r1
      20:  95 00 00 00 00 00 00 00   exit

Add both `callx` instruction encodings to eBPF processor.

By the way, the eBPF Verifier used by Linux kernel currently forbids
indirect calls (it fails when `BPF_SRC(insn->code) != BPF_K`, in
https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/bpf/verifier.c?h=v6.14#n19141
). But other deployments of eBPF may already support this feature.

Author: niooss-ledger

Ghidra/Processors/eBPF/data/languages/eBPF.sinc

@@ -14,7 +14,9 @@ define register offset=0 size=8 [ R0  R1  R2  R3  R4  R5  R6  R7  R8  R9  R10  P
 
 # Instruction encoding: Insop:8, dst_reg:4, src_reg:4, off:16, imm:32 - from lsb to msb
 define token instr(64)
+    llvm_imm_callx_zero=(36, 63)
     imm=(32, 63) signed
+    llvm_reg_callx=(32, 35) # special encoding for callx instruction emitted by LLVM
     off=(16, 31) signed
     src=(12, 15)
     dst=(8, 11)
@@ -31,7 +33,7 @@ define token immtoken(64)
 ;
 
 #To operate with registers
-attach variables [ src dst ] [  R0  R1  R2  R3  R4  R5  R6  R7  R8  R9  R10  _  _  _  _  _  ];
+attach variables [ src dst llvm_reg_callx ] [  R0  R1  R2  R3  R4  R5  R6  R7  R8  R9  R10  _  _  _  _  _  ];
 
 #Arithmetic instructions
 #BPF_ALU64
@@ -356,4 +358,16 @@ disp32: reloc is imm [ reloc = inst_next + imm; ] { export *:4 reloc; }
     call disp32;
 }
 
+# GCC encoding and LLVM 19.1+ encoding
+:CALLX dst is op_alu_jmp_opcode=0x8 & op_alu_jmp_source=1 & op_insn_class=0x5 & src=0 & imm=0 & dst {
+    call [dst];
+}
+
+# LLVM encoding used until LLVM 19.1
+# Introduced in https://github.com/llvm/llvm-project/commit/9a67245d881f4cf89fd8f897ae2cd0bccec49496
+# Modified in https://github.com/llvm/llvm-project/commit/c43ad6c0fddac0bbed5e881801dd2bc2f9eeba2d
+:CALLX llvm_reg_callx is op_alu_jmp_opcode=0x8 & op_alu_jmp_source=1 & op_insn_class=0x5 & dst=0 & src=0 & llvm_imm_callx_zero=0 & llvm_reg_callx {
+    call [llvm_reg_callx];
+}
+
 :EXIT is op_alu_jmp_opcode=0x9 & op_alu_jmp_source=0 & op_insn_class=0x5 { return [*:8 R10]; }