[Sanitizers] Libasan failed to be build with -mthumb and -fno-omit-frame-pointer by GCC

[denis13]

Summary

GCC fails to build libasan with -mthumb and -fno-omit-frame-pointer

../../../../libsanitizer/sanitizer_common/sanitizer_linux.cc: In function 'sanitizer::uptr sanitizer::internal_clone(int (*)(void*), void*, int, void*, int*, void*, int*)':
../../../../libsanitizer/sanitizer_common/sanitizer_linux.cc:1540:1: error: r7 cannot be used in asm here
}

Regarding to arm ABI, r7 register is using for syscall number, r0 for return value, and r1 - r6 for syscall arguments, by the way r7 for arm with thumb mode is using as frame pointer and it looks like we have a conflict in this case. As far as I understood, GCC has a special check inside IRA
if (!TEST_HARD_REG_BIT (crtl->asm_clobbers, HARD_FRAME_POINTER_REGNUM)), which does not allow to have frame pointer register as clobber register.

There is no issue with clang, but because sanitizer's runtime come from llvm compiler-rt, this fix, ,probably, should be inside llvm compiler-rt.

link to gcc bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86755

[m.ostapenko]

Looks good, but let's wait for code owners to accept this as well.

lib/sanitizer_common/sanitizer_linux.cc

1600–1603

Please leave a comment here about why we can't use r7 directly.

[denis13]

lib/sanitizer_common/sanitizer_linux.cc

1600–1603

The problem is -
GCC does not allow to have frame pointer register as clobber register for inline assembler, when flag -fno-omit-frame-pointer is added.

Since arm with thumb mode is using r7 register as frame pointer and at the same time as syscall number register, we got a conflict when is building with -mthumb and -fno-omit-frame-pointer.

But, by the way, there is no issue with clang.

[m.ostapenko]

lib/sanitizer_common/sanitizer_linux.cc

1600–1603

Yeah, I've just meant that it would be nice if you leave this comment in the source code at line 1560.

[m.ostapenko]

Looks good. Will commit this tomorrow if nobody objects.

[denis13]

Ok for trunk ?

[m.ostapenko]

Oh, sorry, forgot about this :(. Please rebase and upload new version.
BTW, please don't forget to add 'llvm-commits' to subscribers so your patch will be visible in ML.

[m.ostapenko]

lgtm

[denis13]

Since no users anymore for this patch, closing it :)

lib/sanitizer_common/sanitizer_linux.cc

uptr internal_clone(int (*fn)(void *), void *child_stack, int flags, void *arg,
   child_stack = (char *)child_stack - 2 * sizeof(unsigned int);
   ((unsigned int *)child_stack)[0] = (uptr)fn;
   ((unsigned int *)child_stack)[1] = (uptr)arg;
   register int r0 __asm__("r0") = flags;
   register void *r1 __asm__("r1") = child_stack;
   register int *r2 __asm__("r2") = parent_tidptr;
   register void *r3 __asm__("r3") = newtls;
   register int *r4 __asm__("r4") = child_tidptr;
-  register int r7 __asm__("r7") = __NR_clone;
+  // GCC does not allow to have frame pointer register as a clobber register
+  // for inline assembler, when flag -fno-omit-frame-pointer is added. By
+  // default, arm with mthumb mode uses r7 as a frame pointer register.
+  register int r8 __asm__("r8") = __NR_clone;
 
 #if __ARM_ARCH > 4 || defined (__ARM_ARCH_4T__)
 # define ARCH_HAS_BX
 #endif
 #if __ARM_ARCH > 4
 # define ARCH_HAS_BLX
 #endif
 
__asm__ __volatile__(
                         *               %r1 = child_stack,
                         *               %r2 = parent_tidptr,
                         *               %r3  = new_tls,
                         *               %r4 = child_tidptr)
                         */
 
                        /* Do the system call */
                        "swi 0x0\n"
+                       "push {r7}\n"
+                       "mov r7, r8\n"
 
                        /* if (%r0 != 0)
                         *   return %r0;
                         */
                        "cmp r0, #0\n"
                        "bne 1f\n"
 
                        /* In the child, now. Call "fn(arg)". */
                        "ldr r0, [sp, #4]\n"
                        "ldr ip, [sp], #8\n"
                        BLX(ip)
                        /* Call _exit(%r0). */
                        "mov r7, %7\n"
                        "swi 0x0\n"
                        "1:\n"
+                       "pop {r7}\n"
                        "mov %0, r0\n"
                        : "=r"(res)
-                       : "r"(r0), "r"(r1), "r"(r2), "r"(r3), "r"(r4), "r"(r7),
+                       : "r"(r0), "r"(r1), "r"(r2), "r"(r3), "r"(r4), "r"(r8),
                          "i"(__NR_exit)
                        : "memory");
   return res;
 }
 #endif  // defined(__x86_64__) && SANITIZER_LINUX
 
 #if SANITIZER_ANDROID
 #if __ANDROID_API__ < 21