Fixes#2136
This is a fairly tricky edge case to support with FEX.
If execveat is used with AT_EMPTY_PATH then the application can pass an
FD to execve instead of a filename. This includes FDs that have been
deleted from the disk so the child process can't open it by filename
anymore.
To work around this limitation, we need to pass the FD to the new FEX
process and open it directly, similar to how binfmt_misc works with FDs.
The FD will get passed through environment variables, which the new
process will check for and then remove the variable from the
environment.
Lots of prickly edge cases to support here.
Without binfmt_misc:
- Passes the FD to FEXLoader directly.
- Requires duplicating the FD if it has O_CLOEXEC on the FD.
With binfmt_misc:
- Shebang file, pass directly to FEXLoader, just like without binfmt.
- x86 ELF Files, rely on the kernel's binfmt_misc support here.
- Unsupported ELF files, let kernel handle it through binfmt_misc
Argument handling:
- The application can pass in no arguments.
- Means our application configurations were failing to find a config
- Also various checks in the frontend were failing.
- If opened through an FD, find the symlink for that FD for the
application configuration instead.
Side note:
Fixed a performance issue in execve where when we were checking for file
format support. Either ELF or Shebang files, we were reading the /whole/
file upfront. We only need to read a header worth of ELF files, and only
257 bytes if it is potentially a shebang file. Should dramatically
reduce some application's execve times.
Migrates lingering instances of the old logger over to fmt where
applicable. This allows removing some of the old defines and functions.
The only remaining usages of the printf-based variant of the logger is
in Tests/LinuxSyscalls/Syscalls.cpp for the strace handling.
This isn't quite a 100% clean sweep of IWYU.
There are some false positives where clang fails.
Additionally there are still a few missed in the frontend side of things
that I didn't get to
This lets us support the final two flags in binfmt_misc that we needed.
1) Support open_binary
2) Support preserve
1)
We already supported the credentials flag, which implied open_binary, but we weren't handling half of it.
With the open_binary flag, the Linux kernel passes us the executable in an FD instead of as a pathname.
This can be found inside of auxv on startup, inside of AT_EXECFD.
If AT_EXECFD is available then we prioritize using that instead of the pathname passed in.
This fixes a potential permissions issue where an executable is executed without read permissions.
2)
The preserve flag has the Linux kernel preserve the original argv[0] that was passed to the application.
Prior to supporting this flag, the kernel would provide us with a resolved program path.
This can happen in the instance where something like `blah` resolves to `/usr/loca/bin/blah` which
isn't what the user originally typed.
This works around this problem by handing the interpreter both the resolved path and the original typed path.
Alongside open_binary, we can just use the FD passed in instead of the resolved path, this means
we can just drop the argv[0] for the guest (which is the kernel resolved path) and pass through arguments
unmangled.
We do have to make a minor assumption here that if we are using EXECFD that we assume preserve.
It isn't until kernel v5.12 that we can actually check AT_FLAGS to see if that was true.
Only the frontends need to deal with ELF files specifically.
The backend doesn't need to be aware of them at all.
Since the ELF handling is the frontend's responsibility, move all the code to the frontend.
Ryan Houdek