Ryan Houdek 472675d471 FEXLoader: Adds support for execveat with AT_EMPTY_PATH ...

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.

2023-01-23 02:06:50 -08:00

..

Utils

FEXLoader: Adds support for execveat with AT_EMPTY_PATH

2023-01-23 02:06:50 -08:00

CMakeLists.txt

Moves ELF handlers from FEXCore to frontend

2021-06-27 02:07:56 -07:00