uml: improve PTRACE_SYSEMU checking

Make the PTRACE_SYSEMU checking more robust.  It will make sure that system
call numbers are reported correctly.  If there is a problem, it will disable
PTRACE_SYSEMU use and use PTRACE_SYSCALL instead.

Thanks to Balaji G for helping reproduce this problem.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 8 deletions

diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 79471f85eb89..3fc13fa8729d 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -144,9 +144,7 @@ static int stop_ptraced_child(int pid, void *stack, int exitcode,
                 int exit_with = WEXITSTATUS(status);
                 if (exit_with == 2)
                         non_fatal("check_ptrace : child exited with status 2. "
-                                  "Serious trouble happening! Try updating "
-                                  "your host skas patch!\nDisabling SYSEMU "
-                                  "support.");
+                                  "\nDisabling SYSEMU support.\n");
                 non_fatal("check_ptrace : child exited with exitcode %d, while "
                           "expecting %d; status 0x%x\n", exit_with,
                           exitcode, status);
@@ -209,6 +207,7 @@ __uml_setup("nosysemu", nosysemu_cmd_param,
 static void __init check_sysemu(void)
 {
         void *stack;
+        unsigned long regs[MAX_REG_NR];
         int pid, n, status, count=0;
 
         non_fatal("Checking syscall emulation patch for ptrace...");
@@ -225,11 +224,20 @@ static void __init check_sysemu(void)
                 fatal("check_sysemu : expected SIGTRAP, got status = %d",
                       status);
 
-        n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET,
-                   os_getpid());
-        if(n < 0)
-                fatal_perror("check_sysemu : failed to modify system call "
-                             "return");
+        if(ptrace(PTRACE_GETREGS, pid, 0, regs) < 0)
+                fatal_perror("check_sysemu : PTRACE_GETREGS failed");
+        if(PT_SYSCALL_NR(regs) != __NR_getpid){
+                non_fatal("check_sysemu got system call number %d, "
+                          "expected %d...", PT_SYSCALL_NR(regs), __NR_getpid);
+                goto fail;
+        }
+
+        n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET, os_getpid());
+        if(n < 0){
+                non_fatal("check_sysemu : failed to modify system call "
+                          "return");
+                goto fail;
+        }
 
         if (stop_ptraced_child(pid, stack, 0, 0) < 0)
                 goto fail_stopped;