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userland assembly: structure readme
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Ciro Santilli 六四事件 法轮功
268
README.adoc
268
README.adoc
@@ -350,7 +350,7 @@ Lol!
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We can also test our hacked glibc on <<user-mode-simulation>> with:
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....
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./run --userland hello
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./run --userland c/hello
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....
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I just noticed that this is actually a good way to develop glibc for other archs.
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@@ -947,8 +947,7 @@ Therefore, we decided to consolidate other userland tutorials that we had scatte
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Notable userland content included / moving into this repository includes:
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* <<arm-userland-assembly>>
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* <<x86-userland-assembly>>
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* <<userland-assembly>>
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* <<c>>
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* <<cpp>>
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* <<posix>>
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@@ -1183,10 +1182,7 @@ echo "$(./getvar --arch aarch64 --baremetal interactive/prompt --emulator gem5 -
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But just stick to newer and better `VExpress_GEM5_V1` unless you have a good reason to use `RealViewPBX`.
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When doing bare metal programming, it is likely that you will want to learn assembly language basics. Have a look at these tutorials for the userland part:
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* <<x86-userland-assembly>>
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* <<arm-userland-assembly>>
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When doing baremetal programming, it is likely that you will want to learn userland assembly first, see: <<userland-assembly>>.
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For more information on baremetal, see the section: <<baremetal>>.
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@@ -3468,8 +3464,8 @@ Reproduction:
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....
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rm -f "$(./getvar buildroot_target_dir)/etc/ld.so.cache"
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./run --userland hello
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./run --userland hello --qemu-which host
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./run --userland c/hello
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./run --userland c/hello --qemu-which host
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....
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Outcome:
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@@ -11310,6 +11306,154 @@ git -C "$(./getvar buildroot_source_dir)" grep 'depends on BR2_TOOLCHAIN_USES_GL
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One "downside" of glibc is that it exercises much more kernel functionality on its more bloated pre-main init, which breaks user mode C hello worlds more often, see: <<user-mode-simulation-with-glibc>>. I quote "downside" because glibc is actually exposing emulator bugs which we should actually go and fix.
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== C
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Programs under link:userland/c/[] are examples of link:https://en.wikipedia.org/wiki/ANSI_C[ANSI C] programming.
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[[cpp]]
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== C++
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Programs under link:userland/cpp/[] are examples of link:https://en.wikipedia.org/wiki/C%2B%2B#Standardization[ISO C] programming.
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== POSIX
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Programs under link:userland/posix/[] are examples of POSIX C programming.
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What is POSIX:
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* https://stackoverflow.com/questions/1780599/what-is-the-meaning-of-posix/31865755#31865755
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* https://unix.stackexchange.com/questions/11983/what-exactly-is-posix/220877#220877
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== Userland assembly
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Programs under `userland/arch/<arch>/` are examples of userland assembly programming:
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* link:userland/arch/x86_64/[] moved from: https://github.com/cirosantilli/x86-assembly-cheat
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* link:userland/arch/arm/[] moved from: https://github.com/cirosantilli/arm-assembly-cheat
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* link:userland/arch/aarch64/[] moved from: https://github.com/cirosantilli/arm-assembly-cheat
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Like other userland programs, these programs can be run as explained at: <<userland-setup>>.
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This section will document ISA generic ideas. ISA specifics are documented on the following sections:
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* <<x86-userland-assembly>>
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* <<arm-userland-assembly>>
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The first example that you want to run for each arch is:
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....
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./run --userland arch/<arch>/add
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....
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e.g.:
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....
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./run --userland arch/x86_64/add
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....
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Sources:
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* link:userland/arch/x86_64/add.S[]
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* link:userland/arch/arm/add.S[]
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* link:userland/arch/aarch64/add.S[]
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This verifies that the venerable `add` instruction and our setup are working.
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Then, modify that program to make the assertion fail:
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....
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TODO
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....
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and then watch the assertion fail:
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....
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./build-userland
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./run --userland arch/x86_64/add
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....
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with error message:
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....
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TODO
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....
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Notice how we give the actual assembly line number where the failing assert was!
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=== User vs system assembly
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By "userland assembly", we mean "the parts of the ISA which can be freely used from userland".
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Most ISAs are divided into a system and userland part, and to running the system part requires elevated privileges such as <<ring0>> in x86.
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One big difference between both is that we can run userland assembly on <<userland-setup>>, which is easier to get running and debug.
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In particular, all the examples outside of <<linux-system-calls,freestanding directories>> link to the C standard library for IO, which is very convenient and portable across host OSes.
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Userland assembly is generally simpler, and a pre-requisite for <<baremetal-setup>>.
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System-land assembly cheats will be put under: <<baremetal-setup>>.
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=== Linux system calls
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The following <<userland-setup>> programs illustrate how to make system calls:
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* x86_64
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** link:userland/arch/x86_64/freestanding/hello.S[]
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** link:userland/arch/x86_64/c/freestanding/hello.c[]
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** link:userland/arch/x86_64/c/freestanding/hello_regvar.c[]
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* arm
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** link:userland/arch/arm/freestanding/hello.S[]
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** link:userland/arch/arm/c/freestanding/hello.c[]
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* aarch64
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** link:userland/arch/aarch64/freestanding/hello.S[]
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** link:userland/arch/aarch64/c/freestanding/hello.c[]
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** link:userland/arch/aarch64/c/freestanding/hello_clobbers.c[]
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Unlike most our other examples, which use the C standard library for portability, examples under `freestanding/` can be only run on Linux.
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Such executables are called freestanding because they don't execute the glibc initialization code, but rather start directly on our custom hand written assembly.
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In order to GDB step debug those executables, you will want to use `--no-continue`, e.g.:
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....
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./run --arch aarch64 --userland arch/aarch64/freestanding/hello --wait-gdb
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./run-gdb --arch aarch64 --no-continue --userland arch/aarch64/freestanding/hello
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....
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You are now left on the very first instruction of our tiny executable!
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Determining the ARM syscall numbers:
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* https://reverseengineering.stackexchange.com/questions/16917/arm64-syscalls-table
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* arm: https://github.com/torvalds/linux/blob/v4.17/arch/arm/tools/syscall.tbl
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* aarch64: https://github.com/torvalds/linux/blob/v4.17/include/uapi/asm-generic/unistd.h
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Determining the ARM syscall interface:
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* https://stackoverflow.com/questions/12946958/what-is-the-interface-for-arm-system-calls-and-where-is-it-defined-in-the-linux
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* https://stackoverflow.com/questions/45742869/linux-syscall-conventions-for-armv8
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Questions about the C inline assembly examples:
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* x86_64
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** https://stackoverflow.com/questions/9506353/how-to-invoke-a-system-call-via-sysenter-in-inline-assembly/54956854#54956854
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* ARM
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** https://stackoverflow.com/questions/10831792/how-to-use-specific-register-in-arm-inline-assembler
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** https://stackoverflow.com/questions/21729497/doing-a-syscall-without-libc-using-arm-inline-assembly
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== x86 userland assembly
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Getting started at: <<userland-assembly>>.
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TODO
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== arm userland assembly
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Getting started at: <<userland-assembly>>.
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TODO
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== Baremetal
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Getting started at: <<baremetal-setup>>
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@@ -12134,91 +12278,6 @@ make CROSS_COMPILE_DIR=/usr/bin
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;
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....
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== C
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Programs under link:userland/c/[] are examples of link:https://en.wikipedia.org/wiki/ANSI_C[ANSI C] programming.
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[[cpp]]
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== C++
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Programs under link:userland/cpp/[] are examples of link:https://en.wikipedia.org/wiki/C%2B%2B#Standardization[ISO C] programming.
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== POSIX
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Programs under link:userland/posix/[] are examples of POSIX C programming.
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What is POSIX:
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* https://stackoverflow.com/questions/1780599/what-is-the-meaning-of-posix/31865755#31865755
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* https://unix.stackexchange.com/questions/11983/what-exactly-is-posix/220877#220877
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== Linux system calls
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The following <<userland-setup>> programs illustrate how to make system calls:
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* x86_64
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** link:userland/arch/x86_64/freestanding/hello.S[]
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** link:userland/arch/x86_64/c/freestanding/hello.c[]
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** link:userland/arch/x86_64/c/freestanding/hello_regvar.c[]
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* arm
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** link:userland/arch/arm/freestanding/hello.S[]
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** link:userland/arch/arm/c/freestanding/hello.c[]
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* aarch64
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** link:userland/arch/aarch64/freestanding/hello.S[]
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** link:userland/arch/aarch64/c/freestanding/hello.c[]
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** link:userland/arch/aarch64/c/freestanding/hello_clobbers.c[]
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Unlike most our other examples, which use the C standard library for portability, examples under `freestanding/` can be only run on Linux.
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Such executables are called freestanding because they don't execute the glibc initialization code, but rather start directly on our custom hand written assembly.
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In order to GDB step debug those executables, you will want to use `--no-continue`, e.g.:
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....
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./run --arch aarch64 --userland arch/aarch64/freestanding/hello --wait-gdb
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./run-gdb --arch aarch64 --no-continue --userland arch/aarch64/freestanding/hello
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....
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Determining the ARM syscall numbers:
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* https://reverseengineering.stackexchange.com/questions/16917/arm64-syscalls-table
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* arm: https://github.com/torvalds/linux/blob/v4.17/arch/arm/tools/syscall.tbl
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* aarch64: https://github.com/torvalds/linux/blob/v4.17/include/uapi/asm-generic/unistd.h
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Determining the ARM syscall interface:
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* https://stackoverflow.com/questions/12946958/what-is-the-interface-for-arm-system-calls-and-where-is-it-defined-in-the-linux
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* https://stackoverflow.com/questions/45742869/linux-syscall-conventions-for-armv8
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Questions about the C inline assembly examples:
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* x86_64
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** https://stackoverflow.com/questions/9506353/how-to-invoke-a-system-call-via-sysenter-in-inline-assembly/54956854#54956854
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* ARM
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** https://stackoverflow.com/questions/10831792/how-to-use-specific-register-in-arm-inline-assembler
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** https://stackoverflow.com/questions/21729497/doing-a-syscall-without-libc-using-arm-inline-assembly
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== x86 userland assembly
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Programs under link:userland/arch/x86_64/[] are examples of x86 userland assembly programming.
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Those examples are progressively being moved out of: https://github.com/cirosantilli/x86-assembly-cheat
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These programs can be run as explained at <<userland-setup>>.
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== arm userland assembly
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Programs under:
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* link:userland/arch/arm/[]
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* link:userland/arch/aarch64/[]
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are examples of ARM userland assembly programming.
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They have been moved out of: https://github.com/cirosantilli/arm-assembly-cheat
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These programs can be run as explained at <<userland-setup>>.
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== Android
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Remember: Android AOSP is a huge undocumented piece of bloatware. It's integration into this repo will likely never be super good.
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@@ -13614,27 +13673,6 @@ git rebase --onto "$next_mainline_revision" "$last_mainline_revision"
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git commit -m "linux: update to ${next_mainline_revision}"
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....
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=== Sanity checks
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Basic C and C++ hello worlds:
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....
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/hello.out
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/hello_cpp.out
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....
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Output:
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....
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hello
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hello cpp
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....
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Sources:
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* link:userland/hello.c[]
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* link:userland/hello_cpp.c[]
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==== rand_check.out
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Print out several parameters that normally change randomly from boot to boot:
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