Module shim 

The exec trampoline: a static binary installed in the alias directory as e, and the record files that tell it what to run.

Without ptrace every tracee has the alias directory as its kernel working directory, so the two-byte path e names the trampoline from any process, whatever length the original path string had. The supervisor writes r<pid> next to it with the plan (see write_record), rewrites the path string to e, lets the exec proceed, and the trampoline execs the real target with the arguments rebuilt from the record and its own, untouched, argv[1..]. A nested launch (see super::nested) takes the trampoline in both modes, exec’d by its full path with ptrace, and its record carries the process setup as well: a complete argument vector, an environment, a directory to enter, session and death-signal flags, seccomp programs, status descriptors, or a message to print and exit with.

In the guest-stub restricted mode every exec takes the trampoline, which arms the guest stub and loads the program in place rather than exec’ing it (see guest_stub); its record then also names the mailbox descriptor, the AT_EXECFN path and the process name, see Trapped.

Outside the guest-stub mode a dynamically linked program of the host’s machine is exec’d through a copy of the trampoline installed as l, the loader, which maps the program where the kernel would and its interpreter after it (see loader_path and ExecPlan).

The binaries under shim/bin/ are built by shim/build.sh from the no_std crate in shim/ and embedded here; one per host architecture.

Structs

Record 🔒

A record under construction.

Trapped

What a record carries in the guest-stub restricted mode.

Constants

SHIM 🔒

SHIM32 🔒

The 32-bit trampoline, which loads the compat ABI’s programs in the guest-stub mode: i386 on x86_64, 32-bit ARM on aarch64.

SHIM_NAME

Name of the trampoline inside the alias directory.

Statics

HAS_32BIT 🔒

Whether the i386 trampoline was installed and runs here.

HAS_LOADER 🔒

Whether the loader was installed and runs here.

INSTALLED 🔒

Functions

dir_bytes

The alias directory as bytes, for building rewritten paths.

has_32bit

Whether the 32-bit trampoline is installed and runs here, which takes a kernel with ia32 emulation, or a CPU and kernel with AArch32.

install

Installs the trampoline and returns the directory holding it.

install_in 🔒

Writes the trampoline into dir and checks it runs there.

loader_bytes 🔒

The loader: a copy of the trampoline shim with the last byte of its role marker rewritten.

loader_path

Path of the installed loader, when it runs here.

path

Path of the installed trampoline.

record_path

Path of the record file of process tgid.

running

Whether thread tid is currently executing the trampoline.

write_record

Writes the record the trampoline will read for process tgid.