Make unprefixed consistently override the system allocator

Signed-off-by: Mads Marquart <mads@marquart.dk>

Author: madsmtm

jemalloc-sys/README.md

@@ -48,11 +48,11 @@ This crate provides following cargo feature flags:
 * `stats` (configure `jemalloc` with `--enable-stats`): Enable statistics
   gathering functionality. See the `jemalloc`'s "`opt.stats_print`" option
   documentation for usage details.
-  
+
 * `debug` (configure `jemalloc` with `--enable-debug`): Enable assertions and
   validation code. This incurs a substantial performance hit, but is very useful
   during application development.
-  
+
 * `background_threads_runtime_support` (enabled by default): enables
   background-threads run-time support when building `jemalloc-sys` on some POSIX
   targets supported by `jemalloc`. Background threads are disabled at run-time
@@ -72,16 +72,25 @@ This crate provides following cargo feature flags:
 * `unprefixed_malloc_on_supported_platforms`: when disabled, configure
   `jemalloc` with `--with-jemalloc-prefix=_rjem_`. Enabling this causes symbols
   like `malloc` to be emitted without a prefix, overriding the ones defined by
-  libc. This usually causes C and C++ code linked in the same program to use
-  `jemalloc` as well. On some platforms prefixes are always used because
-  unprefixing is known to cause segfaults due to allocator mismatches.
-  
+  libc. This usually causes C, Objective-C and C++ code linked in the same
+  program to use `jemalloc` as well. On some platforms prefixes are always used
+  because unprefixing is known to cause segfaults due to allocator mismatches.
+
+  Note that to use this, the `jemalloc-sys` crate must actually be visible to
+  `rustc` (it is not enough to only declare it in `Cargo.toml`). This can be
+  done by adding:
+  ```rust
+  use jemalloc_sys as _;
+  ```
+
+  In your `main.rs`.
+
 * `disable_initial_exec_tls` (disabled by default): when enabled, jemalloc is
-  built with the `--disable-initial-exec-tls` option. It disables the 
-  initial-exec TLS model for jemalloc's internal thread-local storage (on those 
-  platforms that support explicit settings). This can allow jemalloc to be 
+  built with the `--disable-initial-exec-tls` option. It disables the
+  initial-exec TLS model for jemalloc's internal thread-local storage (on those
+  platforms that support explicit settings). This can allow jemalloc to be
   dynamically loaded after program startup (e.g. using dlopen). If you encounter
-  the error `yourlib.so: cannot allocate memory in static TLS block`, you'll 
+  the error `yourlib.so: cannot allocate memory in static TLS block`, you'll
   likely want to enable this.
 
 * `disable_cache_oblivious` (disabled by default): when enabled, jemalloc is
@@ -104,7 +113,7 @@ hyphens `-` are replaced with underscores `_`(see
   variable, the `/etc/malloc.conf` symlink, and the `MALLOC_CONF` environment
   variable (note: this variable might be prefixed as `_RJEM_MALLOC_CONF`). For
   example, to change the default decay time for dirty pages to 30 seconds:
-  
+
   ```
   JEMALLOC_SYS_WITH_MALLOC_CONF=dirty_decay_ms:30000
   ```
@@ -115,17 +124,17 @@ hyphens `-` are replaced with underscores `_`(see
   allocator page size equal to the system page size, so this option need not be
   specified unless the system page size may change between configuration and
   execution, e.g. when cross compiling.
-  
+
 * `JEMALLOC_SYS_WITH_LG_HUGEPAGE=<lg-hugepage>`: Specify the base 2 log of the
   system huge page size. This option is useful when cross compiling, or when
   overriding the default for systems that do not explicitly support huge pages.
-  
-  
+
+
 * `JEMALLOC_SYS_WITH_LG_QUANTUM=<lg-quantum>`: Specify the base 2 log of the
   minimum allocation alignment. jemalloc needs to know the minimum alignment
   that meets the following C standard requirement (quoted from the April 12,
   2011 draft of the C11 standard):
-  
+
   > The pointer returned if the allocation succeeds is suitably aligned so that
   > it may be assigned to a pointer to any type of object with a fundamental
   > alignment requirement and then used to access such an object or an array of

jemalloc-sys/build.rs

@@ -111,10 +111,15 @@ fn main() {
             .iter()
             .any(|i| target.contains(i))
     {
-        warning!(
-            "Unprefixed `malloc` requested on unsupported platform `{}` => using prefixed `malloc`",
-            target
-        );
+        // Apple targets don't support unprefixed, but they do support
+        // overriding (if you do the `zone_register` trick), so no need to
+        // warn there.
+        if !target.contains("apple") {
+            warning!(
+                "Unprefixed `malloc` requested on unsupported platform `{}` => using prefixed `malloc`",
+                target
+            );
+        }
         use_prefix = true;
     }
 

jemalloc-sys/src/env.rs

@@ -21,4 +21,4 @@ pub static NO_BG_THREAD_TARGETS: &[&str] = &["musl"];
 // https://github.com/rust-lang/rust/commit/e3b414d8612314e74e2b0ebde1ed5c6997d28e8d
 // https://github.com/rust-lang/rust/commit/9f3de647326fbe50e0e283b9018ab7c41abccde3
 // https://github.com/rust-lang/rust/commit/ed015456a114ae907a36af80c06f81ea93182a24
-pub static NO_UNPREFIXED_MALLOC_TARGETS: &[&str] = &["android", "dragonfly", "darwin"];
+pub static NO_UNPREFIXED_MALLOC_TARGETS: &[&str] = &["android", "dragonfly", "apple"];

jemalloc-sys/src/lib.rs

@@ -890,3 +890,83 @@ pub type extent_merge_t = unsafe extern "C" fn(
 mod env;
 
 pub use env::*;
+
+// When using the `"unprefixed_malloc_on_supported_platforms"` feature flag,
+// the user wants us to globally override the system allocator.
+//
+// However, since we build `jemalloc` as a static library (an archive), the
+// linker may decide to not care about our overrides if it can't directly see
+// references to the symbols, see the following link for details:
+// <https://maskray.me/blog/2021-06-20-symbol-processing#archive-processing>
+//
+// This is problematic if `jemalloc_sys` is used from a library that by itself
+// doesn't allocate, while invoking other shared libraries that do.
+//
+// Another especially problematic case would be something like the following:
+//
+// ```
+// // Call `malloc` whose symbol is looked up statically.
+// let ptr = libc::malloc(42);
+//
+// // But use a dynamically looked up `free`.
+// let free = libc::dlsym(null_mut(), c"free".as_ptr());
+// let free = transmute::<*mut c_void, unsafe extern "C" fn(*mut c_void)>(free);
+// free(ptr);
+// ```
+//
+// Since if the `malloc` and `free` provided by `jemalloc` end up in different
+// object files in the archive (NOTE: In practice, this is unlikely to be an
+// issue, since `jemalloc.c` contains all the implementations and is compiled
+// as a single object file), the linker would think that only `malloc` was
+// used, and would never load the `free` that we also want (and hence we'd end
+// up executing jemalloc's `malloc` and the system's `free`, which is UB).
+//
+// To avoid this problem, we make sure that all the allocator functions are
+// visible to the linker, such that it will always override all of them.
+//
+// We do this by referencing these symbols in `#[used]` statics, which makes
+// them known to `rustc`, which will reference them in a `symbols.o` stub file
+// that is later passed to the linker. See the following link for details on
+// how this works:
+// <https://github.com/rust-lang/rust/pull/95604>
+
+#[cfg(not(prefixed))]
+mod set_up_statics {
+    use super::*;
+
+    #[used]
+    static USED_MALLOC: unsafe extern "C" fn(usize) -> *mut c_void = malloc;
+    #[used]
+    static USED_CALLOC: unsafe extern "C" fn(usize, usize) -> *mut c_void = calloc;
+    #[used]
+    static USED_POSIX_MEMALIGN: unsafe extern "C" fn(*mut *mut c_void, usize, usize) -> c_int =
+        posix_memalign;
+    #[used]
+    static USED_ALIGNED_ALLOC: unsafe extern "C" fn(usize, usize) -> *mut c_void = aligned_alloc;
+    #[used]
+    static USED_REALLOC: unsafe extern "C" fn(*mut c_void, usize) -> *mut c_void = realloc;
+    #[used]
+    static USED_FREE: unsafe extern "C" fn(*mut c_void) = free;
+}
+
+// On macOS, jemalloc doesn't directly override malloc/free, but instead
+// registers itself with the allocator's zone APIs in a ctor (`zone_register`
+// is marked with `__attribute__((constructor))`).
+//
+// Similarly to above though, for the Mach-O linker to actually consider ctors
+// as "used" when defined in an archive member in a static library, so we need
+// to explicitly reference the function via. Rust's `#[used]`.
+
+#[cfg(all(
+    feature = "unprefixed_malloc_on_supported_platforms",
+    target_vendor = "apple"
+))]
+#[used]
+static USED_ZONE_REGISTER: unsafe extern "C" fn() = {
+    extern "C" {
+        #[cfg_attr(prefixed, link_name = "_rjem_je_zone_register")]
+        #[cfg_attr(not(prefixed), link_name = "je_zone_register")]
+        fn zone_register();
+    }
+    zone_register
+};