How does proot run non-startable programs?

[Maxython]

Hi all.
I recently decided to do an experiment with glibc (the purpose of the experiment is to run it without proot or with minimal use of it). Yes, I know that android does not support glibc and mixing glibc with bionic is a bad idea, but hey, no one forbids compiling it under termux prefix separately from bionic.

In short, I compiled this miracle, but there was one problem that I did not expect. When launching any glibc program, it says Bad system call. I ran the program under strace and the error was shown to be due to syscall set_robust_list.

Using libc.so.6 as an example:

$ strace ./libc.so.6
execve("./libc.so.6", ["./libc.so.6"], 0x7fce2d81e0 /* 23 vars */) = 0
brk(NULL)                               = 0x5c1a99a000
faccessat(AT_FDCWD, "/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x70f0ab3000
set_tid_address(0x70f0ab35a0)           = 29753
set_robust_list(0x70f0ab35b0, 24)       = 485074089392
--- SIGSYS {si_signo=SIGSYS, si_code=SYS_SECCOMP, si_call_addr=0x70f0a97f24, si_syscall=__NR_set_robust_list, si_arch=AUDIT_ARCH_AARCH64} ---
+++ killed by SIGSYS +++
Bad system call

After a little googling, I realized that the linux android kernel does not have this function, then I decided to run glibc programs under proot for fun, and lo and behold, they worked.

After that, I had a question, what does proot do to make non-starting commands run and is it possible to implement this in the command itself?

[ghost]

the bad system call occurred because of seccomp triggered and yes probably missing libc functions. usually non-trusted binaries which was not compiled in Android's libc

proot on the other hand intercepts system calls which allows execution of any binaries and preventing this to happen. this is a common workaround for bad system call errors

[Maxython]

Is it for all syscalls or for specific?

[ghost]

all possible syscalls registered by proot otherwise a program would still run without bad system call error but it would spit function not implemented or the program's actual error

[Maxython]

The result of the experiment: glibc works on termux without proot, the most important thing is to configure it.