Cargo创建空ELF文件

Question

我试图通过使用this answer编写自定义目标.json文件来使用"linker-flavor":"gcc"。我的完整目标.json文件是：

{
  "llvm-target": "avr-atmel-none",
  "cpu": "atmega328p",
  "target-endian": "little",
  "target-pointer-width": "16",
  "os": "none",
  "target-env": "gnu",
  "target-vendor": "unknown",
  "arch": "avr",
  "data-layout": "e-p:16:16:16-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-n8",

  "executables": true,

  "linker": "avr-gcc",
  "linker-flavor": "gcc",
  "pre-link-args": {
    "gcc": ["-Os -mmcu=atmega328p"]
  },
  "exe-suffix": ".elf",
  "post-link-args": {
    "gcc": ["-Wl,--gc-sections"]
  },

  "no-default-libraries": false
}

在没有任何错误消息的情况下运行完cargo build：

$ cargo build --release -v
   Compiling core v0.1.0 (https://github.com/gergoerdi/rust-avr-libcore-mini?rev=adda44aa91ac517aab6915447592ee4cad26564c#adda44aa)
     Running `rustc --crate-name core /home/cactus/.cargo/git/checkouts/rust-avr-libcore-mini-37e279d93a70b45a/adda44a/src/lib.rs --crate-type lib --emit=dep-info,link -C opt-level=3 -C metadata=655bb622dd229da9 -C extra-filename=-655bb622dd229da9 --out-dir /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps --target avr-atmega328p -L dependency=/home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps -L dependency=/home/cactus/prog/rust/avr/chip8-avr/target/release/deps --cap-lints allow`
   Compiling chip8-engine v0.1.0 (https://github.com/gergoerdi/rust-avr-chip8-engine?rev=c6f88737bae4dae0bd6c5c2bbc73737e6dfadfcd#c6f88737)
     Running `rustc --crate-name chip8_engine /home/cactus/.cargo/git/checkouts/rust-avr-chip8-engine-4bce60f3f178d33a/c6f8873/src/lib.rs --crate-type lib --emit=dep-info,link -C opt-level=3 -C metadata=2197ff1f15f697c9 -C extra-filename=-2197ff1f15f697c9 --out-dir /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps --target avr-atmega328p -L dependency=/home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps -L dependency=/home/cactus/prog/rust/avr/chip8-avr/target/release/deps --extern core=/home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libcore-655bb622dd229da9.rlib --cap-lints allow`
   Compiling chip8-avr v0.1.0 (file:///home/cactus/prog/rust/avr/chip8-avr)
     Running `rustc --crate-name chip8_avr src/main.rs --crate-type bin --emit=dep-info,link -C opt-level=3 -C metadata=014a8fed19cbc611 -C extra-filename=-014a8fed19cbc611 --out-dir /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps --target avr-atmega328p -L dependency=/home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps -L dependency=/home/cactus/prog/rust/avr/chip8-avr/target/release/deps --extern chip8_engine=/home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libchip8_engine-2197ff1f15f697c9.rlib --extern core=/home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libcore-655bb622dd229da9.rlib`
    Finished release [optimized] target(s) in 15.99 secs

但是，生成的ELF文件的.text部分为空：

$ avr-objdump -h target/avr-atmega328p/release/chip8-avr.elf 

target/avr-atmega328p/release/chip8-avr.elf:     file format elf32-avr

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000000  00000000  00000000  00000074  2**1
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .data         00000000  00800060  00000000  00000074  2**0
                  CONTENTS, ALLOC, LOAD, DATA
  2 .stab         0000012c  00000000  00000000  00000074  2**2
                  CONTENTS, READONLY, DEBUGGING
  3 .stabstr      0000005d  00000000  00000000  000001a0  2**0
                  CONTENTS, READONLY, DEBUGGING
  4 .comment      00000011  00000000  00000000  000001fd  2**0
                  CONTENTS, READONLY

因此，为了弄清楚发生了什么，我想我应该用一个小的small脚本来替换我的avr-gcc，这个脚本在把它传递给真正的avr-gcc可执行文件之前会记录它的参数。

这向我展示了rustc/cargo试图运行以下命令行来执行链接：

/usr/bin/avr-gcc -Os -mmcu=atmega328p \
  -L /home/cactus/prog/rust/rust-avr/build/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/avr-atmega328p/lib \
  /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/chip8_avr-014a8fed19cbc611.0.o \
  -o /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/chip8_avr-014a8fed19cbc611.elf \
  -Wl,--gc-sections \
  -L /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps -L /home/cactus/prog/rust/avr/chip8-avr/target/release/deps -L /home/cactus/prog/rust/rust-avr/build/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/avr-atmega328p/lib \
  -Wl,-Bstatic /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libchip8_engine-2197ff1f15f697c9.rlib \
  /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libcore-655bb622dd229da9.rlib \
  -Wl,-Bdynamic -Wl,--gc-sections

如果手动运行完全相同的命令，使用完全相同的环境变量，就会得到一个具有正确内容的好的ELF文件(注意它的.text部分不是空的)：

$ /usr/bin/avr-gcc -Os -mmcu=atmega328p -L /home/cactus/prog/rust/rust-avr/build/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/avr-atmega328p/lib /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/chip8_avr-014a8fed19cbc611.0.o -o /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/chip8_avr-014a8fed19cbc611.elf -Wl,--gc-sections -L /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps -L /home/cactus/prog/rust/avr/chip8-avr/target/release/deps -L /home/cactus/prog/rust/rust-avr/build/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/avr-atmega328p/lib -Wl,-Bstatic /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libchip8_engine-2197ff1f15f697c9.rlib /home/cactus/prog/rust/avr/chip8-avr/target/avr-atmega328p/release/deps/libcore-655bb622dd229da9.rlib -Wl,-Bdynamic -Wl,--gc-sections
$ avr-objdump -h target/avr-atmega328p/release/deps/chip8_avr-014a8fed19cbc611.elf  
target/avr-atmega328p/release/deps/chip8_avr-014a8fed19cbc611.elf:     file format elf32-avr

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .data         0000020e  00800100  00001a56  00001af0  2**4
                  CONTENTS, ALLOC, LOAD, DATA
  1 .text         00001a56  00000000  00000000  00000094  2**1
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .bss          000001fa  0080030e  0080030e  00001cfe  2**0
                  ALLOC
  3 .stab         000007ec  00000000  00000000  00001d00  2**2
                  CONTENTS, READONLY, DEBUGGING
  4 .stabstr      000000b0  00000000  00000000  000024ec  2**0
                  CONTENTS, READONLY, DEBUGGING
  5 .comment      00000011  00000000  00000000  0000259c  2**0
                  CONTENTS, READONLY

那么，如果从cargo运行相同的命令会导致一个有效的ELF文件，那么为什么会默默地生成一个无意义的空ELF文件呢？

Answer 1

这是由目标.json文件中的错误引起的；具体来说，这部分：

"pre-link-args": {
  "gcc": ["-Os -mmcu=atmega328p"]
},

参数作为argv直接传递给链接器，因此需要在这里将多个参数拆分为数组的多个元素：

"pre-link-args": {
  "gcc": ["-Os", "-mmcu=atmega328p"]
},

在使用avr-gcc的特殊日志版本时，没有出现此问题的原因是日志只包含所有参数，因此这两种表示没有区别。

至于avr-gcc '-Os -mmcu=atmega328p'创建一个空的.elf文件，似乎这只是不指定任何(有效) -mmcu参数的一个副作用。