Auto merge of #2529 - RalfJung:yesffi, r=RalfJung

re-enable FFI support

https://github.com/tov/libffi-rs/pull/58 landed so the license should no longer be an issue. :)

Fixes https://github.com/rust-lang/miri/issues/2526

6 files changed, 38 insertions, 7 deletions

diff --git a/Cargo.lock b/Cargo.lock
index a7303421808..9df35ec0deb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -309,6 +309,25 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1b03d17f364a3a042d5e5d46b053bbbf82c92c9430c592dd4c064dc6ee997125"
 
 [[package]]
+name = "libffi"
+version = "3.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e454b3efb16fba3b17810ae5e41df02b649e564ab3c5a34b3b93ed07ad287e6"
+dependencies = [
+ "libc",
+ "libffi-sys",
+]
+
+[[package]]
+name = "libffi-sys"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ab4106b7f09d7b87d021334d5618fac1dfcfb824d4c5fe111ff0074dfd242e15"
+dependencies = [
+ "cc",
+]
+
+[[package]]
 name = "libloading"
 version = "0.7.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -392,6 +411,7 @@ dependencies = [
  "getrandom",
  "lazy_static",
  "libc",
+ "libffi",
  "libloading",
  "log",
  "measureme",
diff --git a/Cargo.toml b/Cargo.toml
index c0a217b6411..0a3dfc2a84e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,7 +20,7 @@ doctest = false # and no doc tests
 [dependencies]
 getrandom = { version = "0.2", features = ["std"] }
 env_logger = "0.9"
-#FIXME(miri#2526): libffi = "3.0.0"
+libffi = "3.0.0"
 libloading = "0.7"
 log = "0.4"
 shell-escape = "0.1.4"
diff --git a/README.md b/README.md
index c7a3200dbd9..8e96338a865 100644
--- a/README.md
+++ b/README.md
@@ -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
diff --git a/src/shims/foreign_items.rs b/src/shims/foreign_items.rs
index bd46e4ae80e..b94b6bbb2b8 100644
--- a/src/shims/foreign_items.rs
+++ b/src/shims/foreign_items.rs
@@ -23,7 +23,7 @@ use rustc_target::{
 
 use super::backtrace::EvalContextExt as _;
 use crate::helpers::{convert::Truncate, target_os_is_unix};
-//FIXME(miri#2526): use crate::shims::ffi_support::EvalContextExt as _;
+use crate::shims::ffi_support::EvalContextExt as _;
 use crate::*;
 
 /// Returned by `emulate_foreign_item_by_name`.
@@ -375,9 +375,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             // An Ok(false) here means that the function being called was not exported
             // by the specified `.so` file; we should continue and check if it corresponds to
             // a provided shim.
-            /*FIXME(miri#2526): if this.call_external_c_fct(link_name, dest, args)? {
+            if this.call_external_c_fct(link_name, dest, args)? {
                 return Ok(EmulateByNameResult::NeedsJumping);
-            }*/
+            }
         }
 
         // When adding a new shim, you should follow the following pattern:
diff --git a/src/shims/mod.rs b/src/shims/mod.rs
index 8179d09defe..ee2145db7e1 100644
--- a/src/shims/mod.rs
+++ b/src/shims/mod.rs
@@ -1,7 +1,7 @@
 #![warn(clippy::integer_arithmetic)]
 
 mod backtrace;
-//FIXME(miri#2526): pub mod ffi_support;
+pub mod ffi_support;
 pub mod foreign_items;
 pub mod intrinsics;
 pub mod unix;
diff --git a/tests/compiletest.rs b/tests/compiletest.rs
index 0e492c3eecd..6b5668e2d6c 100644
--- a/tests/compiletest.rs
+++ b/tests/compiletest.rs
@@ -212,8 +212,8 @@ fn main() -> Result<()> {
     ui(Mode::Panic, "tests/panic", WithDependencies)?;
     ui(Mode::Fail { require_patterns: true }, "tests/fail", WithDependencies)?;
     if cfg!(target_os = "linux") {
-        //FIXME(miri#2526): ui(Mode::Pass, "tests/extern-so/pass", WithoutDependencies)?;
-        //FIXME(miri#2526): ui(Mode::Fail { require_patterns: true }, "tests/extern-so/fail", WithDependencies)?;
+        ui(Mode::Pass, "tests/extern-so/pass", WithoutDependencies)?;
+        ui(Mode::Fail { require_patterns: true }, "tests/extern-so/fail", WithDependencies)?;
     }
 
     Ok(())