diff options
Diffstat (limited to '0021-x86-altcall-use-a-union-as-register-type-for-functio.patch')
| -rw-r--r-- | 0021-x86-altcall-use-a-union-as-register-type-for-functio.patch | 141 |
1 files changed, 0 insertions, 141 deletions
diff --git a/0021-x86-altcall-use-a-union-as-register-type-for-functio.patch b/0021-x86-altcall-use-a-union-as-register-type-for-functio.patch deleted file mode 100644 index ab050dd..0000000 --- a/0021-x86-altcall-use-a-union-as-register-type-for-functio.patch +++ /dev/null @@ -1,141 +0,0 @@ -From 1aafe054e7d1efbf8e8482a9cdd4be5753b79e2f Mon Sep 17 00:00:00 2001 -From: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com> -Date: Tue, 27 Feb 2024 14:11:04 +0100 -Subject: [PATCH 21/67] x86/altcall: use a union as register type for function - parameters on clang -MIME-Version: 1.0 -Content-Type: text/plain; charset=UTF-8 -Content-Transfer-Encoding: 8bit - -The current code for alternative calls uses the caller parameter types as the -types for the register variables that serve as function parameters: - -uint8_t foo; -[...] -alternative_call(myfunc, foo); - -Would expand roughly into: - -register unint8_t a1_ asm("rdi") = foo; -register unsigned long a2_ asm("rsi"); -[...] -asm volatile ("call *%c[addr](%%rip)"...); - -However with -O2 clang will generate incorrect code, given the following -example: - -unsigned int func(uint8_t t) -{ - return t; -} - -static void bar(uint8_t b) -{ - int ret_; - register uint8_t di asm("rdi") = b; - register unsigned long si asm("rsi"); - register unsigned long dx asm("rdx"); - register unsigned long cx asm("rcx"); - register unsigned long r8 asm("r8"); - register unsigned long r9 asm("r9"); - register unsigned long r10 asm("r10"); - register unsigned long r11 asm("r11"); - - asm volatile ( "call %c[addr]" - : "+r" (di), "=r" (si), "=r" (dx), - "=r" (cx), "=r" (r8), "=r" (r9), - "=r" (r10), "=r" (r11), "=a" (ret_) - : [addr] "i" (&(func)), "g" (func) - : "memory" ); -} - -void foo(unsigned int a) -{ - bar(a); -} - -Clang generates the following assembly code: - -func: # @func - movl %edi, %eax - retq -foo: # @foo - callq func - retq - -Note the truncation of the unsigned int parameter 'a' of foo() to uint8_t when -passed into bar() is lost. clang doesn't zero extend the parameters in the -callee when required, as the psABI mandates. - -The above can be worked around by using a union when defining the register -variables, so that `di` becomes: - -register union { - uint8_t e; - unsigned long r; -} di asm("rdi") = { .e = b }; - -Which results in following code generated for `foo()`: - -foo: # @foo - movzbl %dil, %edi - callq func - retq - -So the truncation is not longer lost. Apply such workaround only when built -with clang. - -Reported-by: Matthew Grooms <mgrooms@shrew.net> -Link: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277200 -Link: https://github.com/llvm/llvm-project/issues/12579 -Link: https://github.com/llvm/llvm-project/issues/82598 -Signed-off-by: Roger Pau Monné <roger.pau@citrix.com> -Acked-by: Jan Beulich <jbeulich@suse.com> -master commit: 2ce562b2a413cbdb2e1128989ed1722290a27c4e -master date: 2024-02-26 10:18:01 +0100 ---- - xen/arch/x86/include/asm/alternative.h | 25 +++++++++++++++++++++++++ - 1 file changed, 25 insertions(+) - -diff --git a/xen/arch/x86/include/asm/alternative.h b/xen/arch/x86/include/asm/alternative.h -index a7a82c2c03..bcb1dc94f4 100644 ---- a/xen/arch/x86/include/asm/alternative.h -+++ b/xen/arch/x86/include/asm/alternative.h -@@ -167,9 +167,34 @@ extern void alternative_branches(void); - #define ALT_CALL_arg5 "r8" - #define ALT_CALL_arg6 "r9" - -+#ifdef CONFIG_CC_IS_CLANG -+/* -+ * Use a union with an unsigned long in order to prevent clang from -+ * skipping a possible truncation of the value. By using the union any -+ * truncation is carried before the call instruction, in turn covering -+ * for ABI-non-compliance in that the necessary clipping / extension of -+ * the value is supposed to be carried out in the callee. -+ * -+ * Note this behavior is not mandated by the standard, and hence could -+ * stop being a viable workaround, or worse, could cause a different set -+ * of code-generation issues in future clang versions. -+ * -+ * This has been reported upstream: -+ * https://github.com/llvm/llvm-project/issues/12579 -+ * https://github.com/llvm/llvm-project/issues/82598 -+ */ -+#define ALT_CALL_ARG(arg, n) \ -+ register union { \ -+ typeof(arg) e; \ -+ unsigned long r; \ -+ } a ## n ## _ asm ( ALT_CALL_arg ## n ) = { \ -+ .e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \ -+ } -+#else - #define ALT_CALL_ARG(arg, n) \ - register typeof(arg) a ## n ## _ asm ( ALT_CALL_arg ## n ) = \ - ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) -+#endif - #define ALT_CALL_NO_ARG(n) \ - register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) - --- -2.44.0 - |