write/macho: Add support for LC_BUILD_VERSION command (gimli-rs#524)

src/write/macho.rs

@@ -22,6 +22,30 @@ struct SymbolOffsets {
     str_id: Option<StringId>,
 }
 
+/// The customizable portion of a [`macho::BuildVersionCommand`].
+#[derive(Debug, Default, Clone, Copy)]
+#[non_exhaustive] // May want to add the tool list?
+pub struct MachOBuildVersion {
+    /// One of the `PLATFORM_` constants (for example,
+    /// [`object::macho::PLATFORM_MACOS`](macho::PLATFORM_MACOS)).
+    pub platform: u32,
+    /// The minimum OS version, where `X.Y.Z` is encoded in nibbles as
+    /// `xxxx.yy.zz`.
+    pub minos: u32,
+    /// The SDK version as `X.Y.Z`, where `X.Y.Z` is encoded in nibbles as
+    /// `xxxx.yy.zz`.
+    pub sdk: u32,
+}
+
+impl MachOBuildVersion {
+    fn cmdsize(&self) -> u32 {
+        // Same size for both endianness, and we don't have `ntools`.
+        let sz = mem::size_of::<macho::BuildVersionCommand<Endianness>>();
+        debug_assert!(sz <= u32::MAX as usize);
+        sz as u32
+    }
+}
+
 impl<'a> Object<'a> {
     pub(crate) fn macho_set_subsections_via_symbols(&mut self) {
         let flags = match self.flags {
@@ -213,6 +237,12 @@ impl<'a> Object<'a> {
         let mut ncmds = 0;
         let command_offset = offset;
 
+        let build_version_offset = offset;
+        if let Some(version) = &self.macho_build_version {
+            offset += version.cmdsize() as usize;
+            ncmds += 1;
+        }
+
         // Calculate size of segment command and section headers.
         let segment_command_offset = offset;
         let segment_command_len =
@@ -352,6 +382,18 @@ impl<'a> Object<'a> {
             },
         );
 
+        if let Some(version) = &self.macho_build_version {
+            debug_assert_eq!(build_version_offset, buffer.len());
+            buffer.write(&macho::BuildVersionCommand {
+                cmd: U32::new(endian, macho::LC_BUILD_VERSION),
+                cmdsize: U32::new(endian, version.cmdsize()),
+                platform: U32::new(endian, version.platform),
+                minos: U32::new(endian, version.minos),
+                sdk: U32::new(endian, version.sdk),
+                ntools: U32::new(endian, 0),
+            });
+        }
+
         // Write segment command.
         debug_assert_eq!(segment_command_offset, buffer.len());
         macho.write_segment_command(

src/write/mod.rs

@@ -25,6 +25,8 @@ pub mod elf;
 
 #[cfg(feature = "macho")]
 mod macho;
+#[cfg(feature = "macho")]
+pub use macho::MachOBuildVersion;
 
 #[cfg(feature = "pe")]
 pub mod pe;
@@ -70,6 +72,8 @@ pub struct Object<'a> {
     pub mangling: Mangling,
     /// Mach-O "_tlv_bootstrap" symbol.
     tlv_bootstrap: Option<SymbolId>,
+    #[cfg(feature = "macho")]
+    macho_build_version: Option<MachOBuildVersion>,
 }
 
 impl<'a> Object<'a> {
@@ -88,6 +92,8 @@ impl<'a> Object<'a> {
             flags: FileFlags::None,
             mangling: Mangling::default(format, architecture),
             tlv_bootstrap: None,
+            #[cfg(feature = "macho")]
+            macho_build_version: None,
         }
     }
 
@@ -115,6 +121,15 @@ impl<'a> Object<'a> {
         self.mangling = mangling;
     }
 
+    /// Specify information for a Mach-O `LC_BUILD_VERSION` command.
+    ///
+    /// Requires `feature = "macho"`.
+    #[inline]
+    #[cfg(feature = "macho")]
+    pub fn set_macho_build_version(&mut self, info: MachOBuildVersion) {
+        self.macho_build_version = Some(info);
+    }
+
     /// Return the name for a standard segment.
     ///
     /// This will vary based on the file format.