Better description for jit-sealloc option and early check for executable memory.

CMakeLists.txt

@@ -178,7 +178,7 @@ SET(PCRE2_SUPPORT_JIT OFF CACHE BOOL
     "Enable support for Just-in-time compiling.")
 
 SET(PCRE2_SUPPORT_JIT_SEALLOC OFF CACHE BOOL
-    "Enable SELinux compatible execmem allocator in JIT.")
+    "Enable SELinux compatible execmem allocator in JIT (experimental).")
 
 SET(PCRE2GREP_SUPPORT_JIT ON CACHE BOOL
     "Enable use of Just-in-time compiling in pcre2grep.")

README

@@ -164,9 +164,11 @@ library. They are also documented in the pcre2build man page.
   will be a compile time error. If in doubt, use --enable-jit=auto, which
   enables JIT only if the current hardware is supported.
 
-. If you are enabling JIT under SELinux you may also want to add
+. If you are enabling JIT under SELinux environment you may also want to add
-  --enable-jit-sealloc, which enables the use of an execmem allocator in JIT
+  --enable-jit-sealloc, which enables the use of an executable memory allocator
-  that is compatible with SELinux. This has no effect if JIT is not enabled.
+  that is compatible with SELinux. Warning: this allocator is experimental!
+  It does not support fork() operation and may crash when no disk space is
+  available. This option has no effect if JIT is disabled.
 
 . If you do not want to make use of the default support for UTF-8 Unicode
   character strings in the 8-bit library, UTF-16 Unicode character strings in

configure.ac

@@ -161,7 +161,7 @@ fi
 # Handle --enable-jit-sealloc (disabled by default)
 AC_ARG_ENABLE(jit-sealloc,
               AS_HELP_STRING([--enable-jit-sealloc],
-                             [enable SELinux compatible execmem allocator in JIT]),
+                             [enable SELinux compatible execmem allocator in JIT (experimental)]),
               , enable_jit_sealloc=no)
 
 # Handle --disable-pcre2grep-jit (enabled by default)

src/pcre2_jit_compile.c

@@ -13740,12 +13740,33 @@ Returns:        0: success or (*NOJIT) was used
 PCRE2_EXP_DEFN int PCRE2_CALL_CONVENTION
 pcre2_jit_compile(pcre2_code *code, uint32_t options)
 {
+static int executable_allocator_is_working = 0;
+
 pcre2_real_code *re = (pcre2_real_code *)code;
 
 #ifdef SUPPORT_JIT
 executable_functions *functions = (executable_functions *)re->executable_jit;
 #endif
 
+if (executable_allocator_is_working == 0)
+  {
+  /* Checks whether the executable allocator is working. This check
+     might run multiple times in multi-threaded environments, but the result
+     should not be affected by it. */
+  void *ptr = SLJIT_MALLOC_EXEC(32);
+
+  executable_allocator_is_working = -1;
+
+  if (ptr != NULL)
+    {
+    SLJIT_FREE_EXEC(((sljit_u8*)(ptr)) + SLJIT_EXEC_OFFSET(ptr));
+    executable_allocator_is_working = 1;
+    }
+  }
+
+if (executable_allocator_is_working < 0)
+  return PCRE2_ERROR_NOMEMORY;
+
 if (code == NULL)
   return PCRE2_ERROR_NULL;