x86 musl platforms don't like mmap()/ptrace injection

[luckycat889]

Hello,
I have been experimenting with uncommon platforms, and I run into a problem with x86 musl platforms at injection time. There is a neat trick performed during ptrace by directing execution to mmap(), however, on x86/musl the mmap never returns, or returns bad. If I force it to use the text segment swap, it works reliably. I'd love to find out why this happens, and I might experiment, but in the meantime I just log the issue here:
If I run the following patch, it all works

`diff --git a/src/linux/frida-helper-backend.vala b/src/linux/frida-helper-backend.vala
index 9da2152..cca135c 100644
--- a/src/linux/frida-helper-backend.vala
+++ b/src/linux/frida-helper-backend.vala
@@ -1083,7 +1083,7 @@ namespace Frida {
uint64 remote_munmap = 0;
ProcMapsEntry? remote_libc = ProcMapsEntry.find_by_path (pid, local_libc.path);
bool same_libc = remote_libc != null && remote_libc.identity == local_libc.identity;

•                   if (same_libc) {
  

•                   if (false) {
                            remote_mmap = remote_libc.base_address + mmap_offset;
                            remote_munmap = remote_libc.base_address + munmap_offset;
                    }
  

`