BuildID[sha1] of ELF

對於BuildID的解釋, 可以在ELF, Build-ID, is there a utility to recompute it?找到一段說明, 說明如下

I think things weren't very precisely formulated. If a tool changes the build that 
creates the ELF file so that it isn't a "semantically identical" binary anymore 
then it should get a new (recalculated) build-id. But if a tool changes something 
about the file that still results in a "semantically identical" binary then the 
build-id stays the same.

What isn't precisely defined is what "semantically identical binary" means. The 
intention is that it captures everything that a build was made from. So if the 
source files used to generate a binary are different then you expect different 
build-ids, even if the binary code produced might happen to be the same.

This is why when calculating the build-id of a file through a hash algorithm you
 use not just the (allocated) code sections, but also the debuginfo sections 
(which will contain references to the source file names).

But if you then for example strip the debuginfo out (and put it into a separate 
file) then that doesn't change the build-id (the file was still created from the 
same build).

This is also why, even if you knew the precise hashing algorithm used to calculate 
the build-id, you might not be able to recalculate the build-id. Because you might 
be missing some of the original data used in the hashing algorithm to calculate 
the build-id.

Feel free to share this answer with others.

內容簡略的說就是, BuildID是"semantically identical binary", 相同的semantic所build的program才會有相同BuildID.

brook@vista:~/01$ file src/hello
src/hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=91439ef711a19bf3be7774d2c4af35746e098cc8, not stripped
brook@vista:~/01$ readelf -n src/hello

Displaying notes found at file offset 0x00000254 with length 0x00000020:
  Owner                 Data size       Description
  GNU                  0x00000010       NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 2.6.32

Displaying notes found at file offset 0x00000274 with length 0x00000024:
  Owner                 Data size       Description
  GNU                  0x00000014       NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: 91439ef711a19bf3be7774d2c4af35746e098cc8
brook@vista:~/01$ strip src/hello
brook@vista:~/01$ file src/hello
src/hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=91439ef711a19bf3be7774d2c4af35746e098cc8, stripped
brook@vista:~/01$ readelf -n src/hello

Displaying notes found at file offset 0x00000254 with length 0x00000020:
  Owner                 Data size       Description
  GNU                  0x00000010       NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 2.6.32

Displaying notes found at file offset 0x00000274 with length 0x00000024:
  Owner                 Data size       Description
  GNU                  0x00000014       NT_GNU_BUILD_ID (unique build ID bitstring)
    Build ID: 91439ef711a19bf3be7774d2c4af35746e098cc8

brook@vista:~/01$ make clean && make 重build也是會得到相同的BuildID
Making clean in src
...
brook@vista:~/01$ file src/hello
src/hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=91439ef711a19bf3be7774d2c4af35746e098cc8, not stripped

brook@vista:~/01$ echo -e '\n\n\n' >> src/hello.c 即使多了幾行換行, 重build也是會得到相同的BuildID
brook@vista:~/01$ make
make  all-recursive
...
brook@vista:~/01$ file src/hello
src/hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=91439ef711a19bf3be7774d2c4af35746e098cc8, not stripped

參考資料:
• https://stackoverflow.com/questions/41743295/elf-build-id-is-there-a-utility-to-recompute-it, ELF, Build-ID, is there a utility to recompute it?
• https://fedoraproject.org/wiki/Releases/FeatureBuildId, Releases/FeatureBuildId