Optimize process execution on Android

* exec/configure.ac (REENTRANT): Remove option for reentrancy.
(PROGRAM_COUNTER, HAVE_SECCOMP): Define register providing the
program counter and enable seccomp if its headers are available.

* exec/exec.c (write_load_command): Avoid defining unused
variable.
(exec_0): Remove code specific to REENTRANT configurations.

* exec/exec.h (struct exec_tracee) <exec_data, data_size>: New
fields for loader instructions and their size.

* exec/exec1.c (main): Call exec_init before forking.

* exec/mipsel-user.h (ELF_NGREG): Delete definition.
(struct mipsel_regs): Reduce number of gregs to 32, but
introduce separate fields for special registers.

* exec/trace.c (use_seccomp_p): New variable; defile to false if
!HAVE_SECCOMP.
(remove_tracee): Cease providing for non-reentrant
configurations.  Release executable data if present.
(handle_clone_prepare): Likewise.  Resume process with
PTRACE_CONT if seccomp-based interception is enabled.
(handle_clone, check_signal): Resume processes as above.
(handle_exec): Divide into two functions, with only rewriting
the system call and generating instructions for the loader
remaining in the first, and copying such instructions into the
loader's stack removed into a new function, `finish_exec'.
(finish_exec): New function.
(handle_readlinkat, handle_openat): Abolish non-REENTRANT
configurations.
(process_system_call): Divide exec system calls into two phases,
disambiguated by the value of tracee->waiting_for_syscall.  Typo
fixes.  Accommodate syscall-exit-stops where the signal was
initially intercepted by `seccomp_system_call'.
(interesting_syscalls): New array.
(ARRAYELTS): New macro.
(seccomp_system_call, establish_seccomp_filter): New function.
(tracing_execve) [HAVE_SECCOMP]: Establish a seccomp filter if
this is to be enabled.
(after_fork): Provide PTRACE_O_TRACESECCOMP.  Resume process
with PTRACE_CONT if seccomp-based interception is enabled.
(exec_waitpid): Resume process with PTRACE_CONT if seccomp-based
interception is enabled.  Dispatch stops identifying as
PTRACE_EVENT_SECCOMP to `seccomp_system_call'.
(exec_init): Establish whether it is possible to enable seccomp.

exec/configure.ac

@@ -42,11 +42,6 @@ General Public License for more details.
 You should have received a copy of the GNU General Public License
 along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>. */])
 
-AC_ARG_WITH([reentrancy],
-  [AS_HELP_STRING([--with-reentrancy],
-    [Generate library which can be used within a signal handler.])],
-  [AC_DEFINE([REENTRANT], [1])])
-
 AC_USE_SYSTEM_EXTENSIONS
 AC_PROG_CC
 AC_PROG_CPP
@@ -74,9 +69,9 @@ AC_CHECK_FUNC([process_vm_readv],
       ]])])])
 AC_CHECK_HEADERS([sys/param.h sys/uio.h])
 AC_CHECK_MEMBERS([siginfo_t.si_syscall], [], [],
-  [[
+[[
 #include <signal.h>
-  ]])
+]])
 
 AH_BOTTOM([
 #ifdef HAVE_STDBOOL_H
@@ -120,6 +115,7 @@ AH_TEMPLATE([SYSCALL_ARG2_REG], [Define to register holding arg2 to system calls
 AH_TEMPLATE([SYSCALL_ARG3_REG], [Define to register holding arg3 to system calls.])
 AH_TEMPLATE([SYSCALL_RET_REG], [Define to register holding value of system calls.])
 AH_TEMPLATE([STACK_POINTER], [Define to register holding the stack pointer.])
+AH_TEMPLATE([PROGRAM_COUNTER], [Define to register holding the program counter.])
 AH_TEMPLATE([EXEC_SYSCALL], [Define to number of the `exec' system call.])
 AH_TEMPLATE([USER_WORD], [Define to word type used by tracees.])
 AH_TEMPLATE([USER_SWORD], [Define to signed word type used by tracees.])
@@ -134,7 +130,8 @@ AH_TEMPLATE([READLINK_SYSCALL], [Define to number of the `readlink' system call.
 AH_TEMPLATE([READLINKAT_SYSCALL], [Define to number of the `readlinkat' system call.])
 AH_TEMPLATE([OPEN_SYSCALL], [Define to number of the `open' system call.])
 AH_TEMPLATE([OPENAT_SYSCALL], [Define to number of the `openat' system call.])
-AH_TEMPLATE([REENTRANT], [Define to 1 if the library is used within a signal handler.])
+AH_TEMPLATE([HAVE_SECCOMP], [Define to 1 if secure computing filters are available
+to accelerate interception of system calls.])
 
 AC_CANONICAL_HOST
 
@@ -250,6 +247,7 @@ AS_CASE([$host], [x86_64-*linux*],
      AC_DEFINE([SYSCALL_ARG2_REG], [rdx])
      AC_DEFINE([SYSCALL_ARG3_REG], [r10])
      AC_DEFINE([STACK_POINTER], [rsp])
+     AC_DEFINE([PROGRAM_COUNTER], [rip])
      AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
      AC_DEFINE([USER_WORD], [uintptr_t])
      AC_DEFINE([USER_SWORD], [intptr_t])
@@ -283,6 +281,7 @@ AS_CASE([$host], [x86_64-*linux*],
      AC_DEFINE([SYSCALL_ARG2_REG], [edx])
      AC_DEFINE([SYSCALL_ARG3_REG], [esi])
      AC_DEFINE([STACK_POINTER], [esp])
+     AC_DEFINE([PROGRAM_COUNTER], [eip])
      AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
      AC_DEFINE([USER_WORD], [uintptr_t])
      AC_DEFINE([USER_SWORD], [intptr_t])
@@ -314,6 +313,7 @@ AS_CASE([$host], [x86_64-*linux*],
      AC_DEFINE([SYSCALL_ARG2_REG], [[regs[2]]])
      AC_DEFINE([SYSCALL_ARG3_REG], [[regs[3]]])
      AC_DEFINE([STACK_POINTER], [sp])
+     AC_DEFINE([PROGRAM_COUNTER], [pc])
      AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
      AC_DEFINE([USER_WORD], [uintptr_t])
      AC_DEFINE([USER_SWORD], [intptr_t])
@@ -346,6 +346,7 @@ AS_CASE([$host], [x86_64-*linux*],
      AC_DEFINE([SYSCALL_ARG2_REG], [[uregs[2]]])
      AC_DEFINE([SYSCALL_ARG3_REG], [[uregs[3]]])
      AC_DEFINE([STACK_POINTER], [[uregs[13]]])
+     AC_DEFINE([PROGRAM_COUNTER], [[uregs[15]]])
      AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
      AC_DEFINE([USER_WORD], [uintptr_t])
      AC_DEFINE([USER_SWORD], [intptr_t])
@@ -371,6 +372,7 @@ AS_CASE([$host], [x86_64-*linux*],
        AC_DEFINE([SYSCALL_ARG2_REG], [[uregs[2]]])
        AC_DEFINE([SYSCALL_ARG3_REG], [[uregs[3]]])
        AC_DEFINE([STACK_POINTER], [[uregs[13]]])
+       AC_DEFINE([STACK_POINTER], [[uregs[15]]])
        AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
        AC_DEFINE([USER_WORD], [uintptr_t])
        AC_DEFINE([USER_SWORD], [intptr_t])
@@ -402,6 +404,7 @@ AS_CASE([$host], [x86_64-*linux*],
    AC_DEFINE([SYSCALL_ARG2_REG], [[gregs[4]]]) # a2
    AC_DEFINE([SYSCALL_ARG3_REG], [[gregs[5]]]) # a3
    AC_DEFINE([STACK_POINTER], [[gregs[29]]]) # sp
+   AC_DEFINE([PROGRAM_COUNTER], [[cp0_epc]]) # pc
    AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
    AC_DEFINE([USER_WORD], [uintptr_t])
    AC_DEFINE([USER_SWORD], [intptr_t])
@@ -432,6 +435,7 @@ AS_CASE([$host], [x86_64-*linux*],
    AC_DEFINE([SYSCALL_ARG2_REG], [[gregs[4]]]) # a2
    AC_DEFINE([SYSCALL_ARG3_REG], [[gregs[5]]]) # a3
    AC_DEFINE([STACK_POINTER], [[gregs[29]]]) # sp
+   AC_DEFINE([PROGRAM_COUNTER], [[cp0_epc]]) # pc
    AC_DEFINE([EXEC_SYSCALL], [__NR_execve])
    AC_DEFINE([USER_WORD], [uintptr_t])
    AC_DEFINE([USER_SWORD], [intptr_t])
@@ -480,6 +484,12 @@ AC_ARG_VAR([LOADERFLAGS], [Flags used to link the loader.])
 AC_ARG_VAR([ARFLAGS], [Flags for the archiver.])
 AC_ARG_VAR([ASFLAGS], [Flags for the assembler.])
 
+# Search for seccomp headers and declarations.
+AC_CHECK_HEADERS([linux/seccomp.h linux/filter.h],
+  [AC_CHECK_DECLS([SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, SECCOMP_RET_TRACE],
+    [AC_DEFINE([HAVE_SECCOMP], [1])], [],
+    [[#include <linux/seccomp.h>]])])
+
 # Make the assembler optimize for code size.  Don't do this on MIPS,
 # as the assembler code manages branch delays manually.
 

exec/exec.c

@@ -292,7 +292,9 @@ write_load_command (program_header *header, bool use_alternate,
   struct exec_map_command command1;
   USER_WORD start, end;
   bool need_command1;
+#ifndef PAGE_MASK
   static long pagesize;
+#endif /* !PAGE_MASK */
 
   /* First, write the commands necessary to map the specified segment
      itself.
@@ -306,14 +308,14 @@ write_load_command (program_header *header, bool use_alternate,
 #ifdef HAVE_GETPAGESIZE
   if (!pagesize)
     pagesize = getpagesize ();
-#else /* HAVE_GETPAGESIZE */
+#else /* !HAVE_GETPAGESIZE */
   if (!pagesize)
     pagesize = sysconf (_SC_PAGESIZE);
-#endif /* HAVE_GETPAGESIZE */
+#endif /* !HAVE_GETPAGESIZE */
 
 #define PAGE_MASK (~(pagesize - 1))
 #define PAGE_SIZE (pagesize)
-#endif /* PAGE_MASK */
+#endif /* !PAGE_MASK */
 
   start = header->p_vaddr & PAGE_MASK;
   end = ((header->p_vaddr + header->p_filesz
@@ -895,10 +897,6 @@ format_pid (char *in, unsigned int pid)
    with #!; in that case, find the program to open and use that
    instead.
 
-   If REENTRANT is not defined, NAME is actually a buffer of size
-   PATH_MAX + 80.  In that case, copy over the file name actually
-   opened.
-
    Next, read the executable header, and add the necessary memory
    mappings for each file.  Finally, return the action data and its
    size in *SIZE.
@@ -976,11 +974,6 @@ exec_0 (char *name, struct exec_tracee *tracee,
 	  rewrite = buffer1 + link_size;
 	  remaining = buffer1 + sizeof buffer1 - rewrite - 1;
 	  memcpy (rewrite, name, strnlen (name, remaining));
-
-	  /* Replace name with buffer1.  */
-#ifndef REENTRANT
-	  strcpy (name, buffer1);
-#endif /* REENTRANT */
 	}
     }
 

exec/exec.h

@@ -152,6 +152,16 @@ struct exec_tracee
      completion.  */
   USER_WORD sp;
 
+  /* Name of the executable being run.  */
+  char *exec_file;
+
+  /* Pointer to a storage area holding instructions for loading an
+     executable if an `exec' system call is outstanding, or NULL.  */
+  char *exec_data;
+
+  /* Number of bytes in exec_data.  */
+  size_t data_size;
+
   /* The thread ID of this process.  */
   pid_t pid;
 
@@ -162,11 +172,6 @@ struct exec_tracee
   /* Whether or not the tracee has been created but is not yet
      processed by `handle_clone'.  */
   bool new_child : 1;
-
-#ifndef REENTRANT
-  /* Name of the executable being run.  */
-  char *exec_file;
-#endif /* !REENTRANT */
 };
 
 

exec/exec1.c

@@ -42,6 +42,9 @@ main (int argc, char **argv)
   extern char **environ;
   int wstatus;
 
+  /* Provide the file name of the loader.  */
+  exec_init (argv[1]);
+
   pid1 = getpid ();
   pid = fork ();
 
@@ -58,9 +61,6 @@ main (int argc, char **argv)
     }
   else
     {
-      /* Provide the file name of the loader.  */
-      exec_init (argv[1]);
-
       if (after_fork (pid))
 	exit (127);
 

exec/mipsel-user.h

@@ -24,10 +24,6 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include <sys/user.h>
 
-#ifndef ELF_NGREG
-#define ELF_NGREG       45
-#endif /* ELF_NGREG */
-
 
 
 /* This file defines a structure containing user mode general purpose
@@ -36,7 +32,15 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 struct mipsel_regs
 {
   /* General purpose registers.  */
-  uint64_t gregs[ELF_NGREG];
+  uint64_t gregs[32];
+
+  /* Saved special registers. */
+  uint64_t lo;
+  uint64_t hi;
+  uint64_t cp0_epc;
+  uint64_t cp0_badvaddr;
+  uint64_t cp0_status;
+  uint64_t cp0_cause;
 };
 
 #endif /* _MIPSEL_USER_H_ */