72 lines
2.6 KiB
Markdown
72 lines
2.6 KiB
Markdown
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# System calls translation
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The pipeline uses kernel as a source of truth. Unfortunately, all other options proved to be either
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incomplete or imprecise.
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System call names, numbers and entry points are extracted from kernel sources.
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*TODO: consider extracting names, numbers and entry points from vmlinux!*
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System call parameters are extracted from vmlinux DWARF.
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Automatic translation happens between system calls with the same entry point if entry point
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parameters are compatible.
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### Checkout kernel
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Assume the kernel checkout is at `$KERNEL_SRC`.
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### Extract system calls names, numbers and entry points
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This is specific for architectures. The script does extraction for `arm`, `arm64`, `x86`,
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`x86_64` and `riscv64`.
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In addition, the script checks entry point prototypes and marks those without parameters. System
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calls without parameters do not need compatibility checking (and they don't have corresponding
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`__do_sys_*` functions, see "Extract system calls parameters").
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Unfortunately, extracting full info about parameters from prototypes is a bad idea. Most
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architectures have specific calling conventions for system calls, so even the number of parameters
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may vary. But, hopefully, "no parameters" means the same thing everywhere.
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```./extract_syscalls_from_kernel_src.py $KERNEL_SRC/common > ./kernel_syscalls.json```
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### Extract system calls parameters
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Entry point `sys_foo` is a function that gets system call parameters in a form of `struct ptregs*`.
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`SYSCALL_DEFINEx` macro unpacks parameters and calls implementation function `__do_sys_foo` which
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receives parameters normally.
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`__do_sys_foo` functions are usually inlined and do not exist in symbols table. However, DWARF
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should still have these functions.
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To get parameters info in our common .json API form, do:
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Build kernel for arch with debug info. Assume this is `VMLINUX_RISCV64`.
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Extract the list of `__do_sys_*` functions for arch:
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```./gen_kernel_syscalls_symbols.py riscv64 ./kernel_syscalls.json > ./symbols_riscv64.txt```
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Use `dwarf_reader` to get .json API info for these functions:
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```dwarf_reader --filter=symbols_riscv64.txt VMLINUX_RISCV64 > ./kernel_api_riscv64.json```
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### Generate system calls translation
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Now there should be files like:
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```
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kernel_syscalls.json (common system calls info)
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kernel_api_riscv64.json (src arch system calls API)
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kernel_api_x86_64.json (dst arch system calls API)
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custom_syscalls.json (translation exceptions)
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```
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Run the script to generate code for translation function:
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```
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gen_kernel_syscalls_translation.py riscv64 x86_64 \
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> kernel_api/gen_syscall_emulation_riscv64_to_x86_64-inl.h
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```
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