[clang] [Darwin][Driver][clang] Prioritise command line args over `DEFAULT_SYSROOT` (PR #115993)

[Carlo Cabrera via cfe-commits]

https://github.com/carlocab updated https://github.com/llvm/llvm-project/pull/115993

>From b072008a5b1a3bd697e3f9efc091bcddfbcc3547 Mon Sep 17 00:00:00 2001
From: Carlo Cabrera <github at carlo.cab>
Date: Wed, 13 Nov 2024 13:09:59 +0800
Subject: [PATCH] [Darwin][Driver][clang] Prioritise command line flags over
 `DEFAULT_SYSROOT`

If a toolchain is configured with `DEFAULT_SYSROOT`, then this could
result in an unintended value for `-syslibroot` being passed to the
linker if the user manually sets `-isysroot` or `SDKROOT`.

Let's fix this by prioritising command line flags when determining
`-syslibroot` before checking `getSysRoot`.

Downstream bug report: https://github.com/Homebrew/homebrew-core/issues/197277

Co-authored-by: Bo Anderson <mail at boanderson.me>
---
 clang/lib/Driver/ToolChains/Darwin.cpp | 10 +++++++---
 clang/test/Driver/sysroot.c            |  3 +--
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
index 87380869f6fdab..cdb6d21a0148b6 100644
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -430,13 +430,17 @@ void darwin::Linker::AddLinkArgs(Compilation &C, const ArgList &Args,
 
   // Give --sysroot= preference, over the Apple specific behavior to also use
   // --isysroot as the syslibroot.
-  StringRef sysroot = C.getSysRoot();
-  if (sysroot != "") {
+  // We check `OPT__sysroot_EQ` directly instead of `getSysRoot` to make sure we
+  // prioritise command line arguments over configuration of `DEFAULT_SYSROOT`.
+  if (const Arg *A = Args.getLastArg(options::OPT__sysroot_EQ)) {
     CmdArgs.push_back("-syslibroot");
-    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
+    CmdArgs.push_back(A->getValue());
   } else if (const Arg *A = Args.getLastArg(options::OPT_isysroot)) {
     CmdArgs.push_back("-syslibroot");
     CmdArgs.push_back(A->getValue());
+  } else if (StringRef sysroot = C.getSysRoot(); sysroot != "") {
+    CmdArgs.push_back("-syslibroot");
+    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
   }
 
   Args.AddLastArg(CmdArgs, options::OPT_twolevel__namespace);
diff --git a/clang/test/Driver/sysroot.c b/clang/test/Driver/sysroot.c
index 85da2499090af1..3080f76e031686 100644
--- a/clang/test/Driver/sysroot.c
+++ b/clang/test/Driver/sysroot.c
@@ -4,10 +4,9 @@
 // CHECK-SYSROOTEQ: "-cc1"{{.*}} "-isysroot" "{{[^"]*}}/FOO"
 
 // Apple Darwin uses -isysroot as the syslib root, too.
-// We pass --sysroot="" to defeat any -DDEFAULT_SYSROOT parameter.
 // RUN: touch %t2.o
 // RUN: %clang -target i386-apple-darwin10 \
-// RUN:   -isysroot /FOO --sysroot="" -### %t2.o 2> %t2
+// RUN:   -isysroot /FOO -### %t2.o 2> %t2
 // RUN: FileCheck --check-prefix=CHECK-APPLE-ISYSROOT < %t2 %s
 // CHECK-APPLE-ISYSROOT: "-arch" "i386"{{.*}} "-syslibroot" "{{[^"]*}}/FOO"