[Qemu-devel] [PULL v2 21/24] linux-user: Provide safe_syscall for arm

[riku . voipio]

From: Richard Henderson <address@hidden>

Reviewed-by: Peter Maydell <address@hidden>
Signed-off-by: Richard Henderson <address@hidden>
Signed-off-by: Riku Voipio <address@hidden>
---
 linux-user/host/arm/hostdep.h          | 23 +++++++++
 linux-user/host/arm/safe-syscall.inc.S | 90 ++++++++++++++++++++++++++++++++++
 2 files changed, 113 insertions(+)
 create mode 100644 linux-user/host/arm/safe-syscall.inc.S

diff --git a/linux-user/host/arm/hostdep.h b/linux-user/host/arm/hostdep.h
index 7609bf5..8e1ff2f 100644
--- a/linux-user/host/arm/hostdep.h
+++ b/linux-user/host/arm/hostdep.h
@@ -12,4 +12,27 @@
 #ifndef QEMU_HOSTDEP_H
 #define QEMU_HOSTDEP_H
 
+/* We have a safe-syscall.inc.S */
+#define HAVE_SAFE_SYSCALL
+
+#ifndef __ASSEMBLER__
+
+/* These are defined by the safe-syscall.inc.S file */
+extern char safe_syscall_start[];
+extern char safe_syscall_end[];
+
+/* Adjust the signal context to rewind out of safe-syscall if we're in it */
+static inline void rewind_if_in_safe_syscall(void *puc)
+{
+    struct ucontext *uc = puc;
+    unsigned long *pcreg = &uc->uc_mcontext.arm_pc;
+
+    if (*pcreg > (uintptr_t)safe_syscall_start
+        && *pcreg < (uintptr_t)safe_syscall_end) {
+        *pcreg = (uintptr_t)safe_syscall_start;
+    }
+}
+
+#endif /* __ASSEMBLER__ */
+
 #endif
diff --git a/linux-user/host/arm/safe-syscall.inc.S 
b/linux-user/host/arm/safe-syscall.inc.S
new file mode 100644
index 0000000..88c4958
--- /dev/null
+++ b/linux-user/host/arm/safe-syscall.inc.S
@@ -0,0 +1,90 @@
+/*
+ * safe-syscall.inc.S : host-specific assembly fragment
+ * to handle signals occurring at the same time as system calls.
+ * This is intended to be included by linux-user/safe-syscall.S
+ *
+ * Written by Richard Henderson <address@hidden>
+ * Copyright (C) 2016 Red Hat, Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+       .global safe_syscall_base
+       .global safe_syscall_start
+       .global safe_syscall_end
+       .type   safe_syscall_base, %function
+
+       .cfi_sections   .debug_frame
+
+       .text
+       .syntax unified
+       .arm
+       .align 2
+
+       /* This is the entry point for making a system call. The calling
+        * convention here is that of a C varargs function with the
+        * first argument an 'int *' to the signal_pending flag, the
+        * second one the system call number (as a 'long'), and all further
+        * arguments being syscall arguments (also 'long').
+        * We return a long which is the syscall's return value, which
+        * may be negative-errno on failure. Conversion to the
+        * -1-and-errno-set convention is done by the calling wrapper.
+        */
+safe_syscall_base:
+       .fnstart
+       .cfi_startproc
+       mov     r12, sp                 /* save entry stack */
+       push    { r4, r5, r6, r7, r8, lr }
+       .save   { r4, r5, r6, r7, r8, lr }
+       .cfi_adjust_cfa_offset 24
+       .cfi_rel_offset r4, 0
+       .cfi_rel_offset r5, 4
+       .cfi_rel_offset r6, 8
+       .cfi_rel_offset r7, 12
+       .cfi_rel_offset r8, 16
+       .cfi_rel_offset lr, 20
+
+       /* The syscall calling convention isn't the same as the C one:
+        * we enter with r0 == *signal_pending
+        *               r1 == syscall number
+        *               r2, r3, [sp+0] ... [sp+12] == syscall arguments
+        *               and return the result in r0
+        * and the syscall instruction needs
+        *               r7 == syscall number
+        *               r0 ... r6 == syscall arguments
+        *               and returns the result in r0
+        * Shuffle everything around appropriately.
+        * Note the 16 bytes that we pushed to save registers.
+        */
+       mov     r8, r0                  /* copy signal_pending */
+       mov     r7, r1                  /* syscall number */
+       mov     r0, r2                  /* syscall args */
+       mov     r1, r3
+       ldm     r12, { r2, r3, r4, r5, r6 }
+
+       /* This next sequence of code works in conjunction with the
+        * rewind_if_safe_syscall_function(). If a signal is taken
+        * and the interrupted PC is anywhere between 'safe_syscall_start'
+        * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
+        * The code sequence must therefore be able to cope with this, and
+        * the syscall instruction must be the final one in the sequence.
+        */
+safe_syscall_start:
+       /* if signal_pending is non-zero, don't do the call */
+       ldr     r12, [r8]               /* signal_pending */
+       tst     r12, r12
+       bne     1f
+       swi     0
+safe_syscall_end:
+       /* code path for having successfully executed the syscall */
+       pop     { r4, r5, r6, r7, r8, pc }
+
+1:
+       /* code path when we didn't execute the syscall */
+       ldr     r0, =-TARGET_ERESTARTSYS
+       pop     { r4, r5, r6, r7, r8, pc }
+       .fnend
+       .cfi_endproc
+
+       .size   safe_syscall_base, .-safe_syscall_base
-- 
2.1.4