Auto merge of #2363 - emarteca:int-function-args-returns, r=oli-obk

Adding support for external C functions that have integer (or empty) args and/or returns

Starts addressing `@https://github.com/rust-lang/miri/issues/11`

### Implementation
Adding support for calling external C functions that have any number of integer arguments (types of integers: `i8`, `i16`, `i32`, `i64`, `u8`, `u16`, `u32`, `u64`) and an integer return type (or `void`).
As suggested in `@https://github.com/rust-lang/miri/issues/11,` the [`libffi` crate](https://docs.rs/libffi/latest/libffi/index.html) is used to dispatch the calls to external C functions.

#### Modifications
Main modifications are to:
* [helper.rs](https://github.com/emarteca/miri/blob/int-function-args-returns/src/helpers.rs) : adding a function `call_and_add_external_c_fct_to_context` to read the code pointer to the external C function, dispatch the call, and save the return in MIRI internal memory. Handles all conversions between MIRI and C values (using some macros, also defined in this file).
* [foreign_items.rs](https://github.com/emarteca/miri/blob/int-function-args-returns/src/shims/foreign_items.rs) : handles the calling of `call_and_add_external_c_fct_to_context` in [helper.rs](https://github.com/emarteca/miri/blob/int-function-args-returns/src/helpers.rs) when a foreign item is encountered. Also adds some structs to model C representations of arguments, and the signature of the external C call.

### Testing
Adds tests for the following external functions which are now supported:
* [int tests](https://github.com/emarteca/miri/blob/int-function-args-returns/tests/pass/external_C/int_c_tests.rs):
     - adds 2 to a provided int (no type of int specified, so autocasts)
     - takes the sum of its 12 arguments (tests stack spill)
     - adds 3 to a 16 bit int
     - adds an `i16` to an `i64`
     - returns -10 as an unsigned int
* [void tests](https://github.com/emarteca/miri/blob/int-function-args-returns/tests/pass/external_C/print_from_c.rs)
     - void function that prints from C

### Code review
The code in this PR was reviewed by `@maurer` on [another fork](maurer#1) -- thanks!

Author: bors

.gitignore

@@ -2,10 +2,18 @@ target
 /doc
 tex/*/out
 *.dot
+*.out
 *.rs.bk
 .vscode
 *.mm_profdata
 perf.data
 perf.data.old
 flamegraph.svg
+<<<<<<< HEAD
+=======
+<<<<<<< HEAD
+tests/extern-so/libtestlib.so
+=======
+>>>>>>> master
+>>>>>>> 58ba05a0 (C FFI support for functions with int args and returns)
 .auto-*

Cargo.lock

@@ -20,6 +20,8 @@ doctest = false # and no doc tests
 [dependencies]
 getrandom = { version = "0.2", features = ["std"] }
 env_logger = "0.9"
+libffi = "3.0.0"
+libloading = "0.7"
 log = "0.4"
 shell-escape = "0.1.4"
 rand = "0.8"

Cargo.toml

@@ -346,6 +346,17 @@ to Miri failing to detect cases of undefined behavior in a program.
   this flag is **unsound**.
 * `-Zmiri-disable-weak-memory-emulation` disables the emulation of some C++11 weak
   memory effects.
+* `-Zmiri-extern-so-file=<path to a shared object file>` is an experimental flag for providing support
+  for FFI calls. Functions not provided by that file are still executed via the usual Miri shims.
+  **WARNING**: If an invalid/incorrect `.so` file is specified, this can cause undefined behaviour in Miri itself!
+  And of course, Miri cannot do any checks on the actions taken by the external code.
+  Note that Miri has its own handling of file descriptors, so if you want to replace *some* functions
+  working on file descriptors, you will have to replace *all* of them, or the two kinds of
+  file descriptors will be mixed up.
+  This is **work in progress**; currently, only integer arguments and return values are
+  supported (and no, pointer/integer casts to work around this limitation will not work;
+  they will fail horribly).
+  Follow [the discussion on supporting other types](https://github.com/rust-lang/miri/issues/2365). 
 * `-Zmiri-measureme=<name>` enables `measureme` profiling for the interpreted program.
    This can be used to find which parts of your program are executing slowly under Miri.
    The profile is written out to a file with the prefix `<name>`, and can be processed

README.md

@@ -0,0 +1,6 @@
+fn main() {
+    // Re-export the TARGET environment variable so it can
+    // be accessed by miri.
+    let target = std::env::var("TARGET").unwrap();
+    println!("cargo:rustc-env=TARGET={:?}", target);
+}

build.rs

@@ -108,6 +108,7 @@ esac
 
 ## Prepare the environment
 # Determine some toolchain properties
+# export the target so its available in miri
 TARGET=$(rustc +$TOOLCHAIN --version --verbose | grep "^host:" | cut -d ' ' -f 2)
 SYSROOT=$(rustc +$TOOLCHAIN --print sysroot)
 LIBDIR=$SYSROOT/lib/rustlib/$TARGET/lib

miri

@@ -530,6 +530,19 @@ fn main() {
                 "full" => BacktraceStyle::Full,
                 _ => show_error!("-Zmiri-backtrace may only be 0, 1, or full"),
             };
+        } else if let Some(param) = arg.strip_prefix("-Zmiri-extern-so-file=") {
+            let filename = param.to_string();
+            if std::path::Path::new(&filename).exists() {
+                if let Some(other_filename) = miri_config.external_so_file {
+                    panic!(
+                        "-Zmiri-extern-so-file external SO file is already set to {}",
+                        other_filename.display()
+                    );
+                }
+                miri_config.external_so_file = Some(filename.into());
+            } else {
+                panic!("-Zmiri-extern-so-file path {} does not exist", filename);
+            }
         } else {
             // Forward to rustc.
             rustc_args.push(arg);

src/bin/miri.rs

@@ -3,6 +3,7 @@
 use std::ffi::{OsStr, OsString};
 use std::iter;
 use std::panic::{self, AssertUnwindSafe};
+use std::path::PathBuf;
 use std::thread;
 
 use log::info;
@@ -128,6 +129,9 @@ pub struct MiriConfig {
     pub report_progress: Option<u32>,
     /// Whether Stacked Borrows retagging should recurse into fields of datatypes.
     pub retag_fields: bool,
+    /// The location of a shared object file to load when calling external functions
+    /// FIXME! consider allowing users to specify paths to multiple SO files, or to a directory
+    pub external_so_file: Option<PathBuf>,
 }
 
 impl Default for MiriConfig {
@@ -159,6 +163,7 @@ impl Default for MiriConfig {
             preemption_rate: 0.01, // 1%
             report_progress: None,
             retag_fields: false,
+            external_so_file: None,
         }
     }
 }

src/eval.rs

@@ -358,10 +358,14 @@ pub struct Evaluator<'mir, 'tcx> {
     pub(crate) report_progress: Option<u32>,
     /// The number of blocks that passed since the last progress report.
     pub(crate) since_progress_report: u32,
+
+    /// Handle of the optional shared object file for external functions.
+    pub external_so_lib: Option<(libloading::Library, std::path::PathBuf)>,
 }
 
 impl<'mir, 'tcx> Evaluator<'mir, 'tcx> {
     pub(crate) fn new(config: &MiriConfig, layout_cx: LayoutCx<'tcx, TyCtxt<'tcx>>) -> Self {
+        let target_triple = &layout_cx.tcx.sess.opts.target_triple.to_string();
         let local_crates = helpers::get_local_crates(layout_cx.tcx);
         let layouts =
             PrimitiveLayouts::new(layout_cx).expect("Couldn't get layouts of primitive types");
@@ -412,6 +416,24 @@ impl<'mir, 'tcx> Evaluator<'mir, 'tcx> {
             preemption_rate: config.preemption_rate,
             report_progress: config.report_progress,
             since_progress_report: 0,
+            external_so_lib: config.external_so_file.as_ref().map(|lib_file_path| {
+                // Check if host target == the session target.
+                if option_env!("TARGET") == Some(target_triple) {
+                    panic!(
+                        "calling external C functions in linked .so file requires target and host to be the same"
+                    );
+                }
+                // Note: it is the user's responsibility to provide a correct SO file.
+                // WATCH OUT: If an invalid/incorrect SO file is specified, this can cause
+                // undefined behaviour in Miri itself!
+                (
+                    unsafe {
+                        libloading::Library::new(lib_file_path)
+                            .expect("Failed to read specified shared object file")
+                    },
+                    lib_file_path.clone(),
+                )
+            }),
         }
     }