Ok so, it's been a year or so since I was buggering around with the ELF internals (I wrote a simpler header in assembly so I could make a ridiculously small binary...). Let's take a look at an ELF program. If you run `readelf -l $(which gcc)` you get a bunch of output, among that is:

alx@foo:~$ readelf -l $(which gcc)

Elf file type is EXEC (Executable file)
Entry point 0x467de0
There are 10 program headers, starting at offset 64

Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
PHDR 0x0000000000000040 0x0000000000400040 0x0000000000400040
0x0000000000000230 0x0000000000000230 R 0x8
INTERP 0x0000000000000270 0x0000000000400270 0x0000000000400270
0x000000000000001c 0x000000000000001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x00000000000fa8f4 0x00000000000fa8f4 R E 0x200000

you can see that in the ELF header is a field called "INTERP" that requests the loader. This is because the program has been compiled with the -fPIE flag, which requests a "Position Independent Executable". This means that each section in the code has been compiled so that they don't expect a set position in memory for the other sections. In other words, you can't just run it on a UNIX computer and expect it to work, it relies on another library, to load each section, and tell the other sections where to load it.

The problem with this is that the musl loader (I don't have my x200 available right now to copy some output from it to illustrate the difference) is usually at a different place in memory. What this means is that when the program is run, the ELF loader tries to find the program interpreter to execute the program, because musl libc's program interpreter is at a different place and name in the filesystem hierarchy, it fails to execute the program, and returns "Not a valid executable".

Now you would think a naive solution would be to symlink the musl libc loader to the expected position in the filesystem hierarchy. The problem with this is illustrated when you look at the other dependencies and symbols exported in the program. Let's have a look:

alx@foo:~$ readelf -s $(which gcc)

Symbol table '.dynsym' contains 153 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __strcat_chk@GLIBC_2.3.4 (2)
2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __uflow@GLIBC_2.2.5 (3)
3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND mkstemps@GLIBC_2.11 (4)
4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND getenv@GLIBC_2.2.5 (3)
5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND dl_iterate_phdr@GLIBC_2.2.5 (3)
6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __snprintf_chk@GLIBC_2.3.4 (2)
7: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __pthread_key_create
8: 0000000000000000 0 FUNC GLOBAL DEFAULT UND putchar@GLIBC_2.2.5 (3)
9: 0000000000000000 0 FUNC GLOBAL DEFAULT UND strcasecmp@GLIBC_2.2.5 (3)

As you can see, the program not only expects a GNU program interpreter, but the symbols the program has been linked against expect GLIBC_2.2.5 version numbers as part of the exported symbols (Although I cannot recall if this causes a problem or not, memory says it does, but you'd be better off reading the ELF specification at this point, which you can find here: https://refspecs.linuxfoundation.org/LSB_2.1.0/LSB-Core-gene...). So the ultimate result of trying to run this program on a GNU LibC system is that it fails to run, because the symbols are 'missing'. On top of this, you can see with `readelf -d` that it relies on the libc library:

alx@foo:~$ readelf -d $(which gcc)

Dynamic section at offset 0xfddd8 contains 25 entries:
Tag Type Name/Value
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x0000000000000001 (NEEDED) Shared library: [ld-linux-x86-64.so.2]
0x000000000000000c (INIT) 0x4026a8

Unfortunately for us, the libc.so.6 binary produced by the GNU system is also symbolically incompatible with the one produced by musl, also GNU LibC defines some functions and symbols that are not in the C standard. The ultimate result of this is that you need to link statically against libc, and against the program loader, for this binary to have a chance at running on a musl system.