Initialize tunable list with the GLIBC_TUNABLES environment variable

Read tunables values from the users using the GLIBC_TUNABLES
environment variable.  The value of this variable is a colon-separated
list of name=value pairs.  So a typical string would look like this:

GLIBC_TUNABLES=glibc.malloc.mmap_threshold=2048:glibc.malloc.trim_threshold=1024

	* config.make.in (have-loop-to-function): Define.
	* elf/Makefile (CFLAGS-dl-tunables.c): Add
	-fno-tree-loop-distribute-patterns.
	* elf/dl-tunables.c: Include libc-internals.h.
	(GLIBC_TUNABLES): New macro.
	(tunables_strdup): New function.
	(parse_tunables): New function.
	(min_strlen): New function.
	(__tunables_init): Use the new functions and macro.
	(disable_tunable): Disable tunable from GLIBC_TUNABLES.
	* malloc/tst-malloc-usable-tunables.c: New test case.
	* malloc/tst-malloc-usable-static-tunables.c: New test case.
	* malloc/Makefile (tests, tests-static): Add tests.

2 files changed, 134 insertions, 1 deletions

diff --git a/elf/Makefile b/elf/Makefile
index de28d99..3cda2c9 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -38,6 +38,12 @@ endif
 
 ifeq (yes,$(have-tunables))
 dl-routines += dl-tunables
+
+# Make sure that the compiler does not insert any library calls in tunables
+# code paths.
+ifeq (yes,$(have-loop-to-function))
+CFLAGS-dl-tunables.c = -fno-tree-loop-distribute-patterns
+endif
 endif
 
 all-dl-routines = $(dl-routines) $(sysdep-dl-routines)
diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index 472747b..8d69140 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -30,7 +30,10 @@
 #define TUNABLES_INTERNAL 1
 #include "dl-tunables.h"
 
-/* Compare environment names, bounded by the name hardcoded in glibc.  */
+#define GLIBC_TUNABLES "GLIBC_TUNABLES"
+
+/* Compare environment or tunable names, bounded by the name hardcoded in
+   glibc.  */
 static bool
 is_name (const char *orig, const char *envname)
 {
@@ -45,6 +48,29 @@ is_name (const char *orig, const char *envname)
     return false;
 }
 
+static char *
+tunables_strdup (const char *in)
+{
+  size_t i = 0;
+
+  while (in[i++] != '\0');
+  char *out = __sbrk (i);
+
+  /* FIXME: In reality if the allocation fails, __sbrk will crash attempting to
+     set the thread-local errno since the TCB has not yet been set up.  This
+     needs to be fixed with an __sbrk implementation that does not set
+     errno.  */
+  if (out == (void *)-1)
+    return NULL;
+
+  i--;
+
+  while (i-- > 0)
+    out[i] = in[i];
+
+  return out;
+}
+
 static char **
 get_next_env (char **envp, char **name, size_t *namelen, char **val)
 {
@@ -218,6 +244,82 @@ tunable_initialize (tunable_t *cur, const char *strval)
     }
 }
 
+static void
+parse_tunables (char *tunestr)
+{
+  if (tunestr == NULL || *tunestr == '\0')
+    return;
+
+  char *p = tunestr;
+
+  while (true)
+    {
+      char *name = p;
+      size_t len = 0;
+
+      /* First, find where the name ends.  */
+      while (p[len] != '=' && p[len] != ':' && p[len] != '\0')
+	len++;
+
+      /* If we reach the end of the string before getting a valid name-value
+	 pair, bail out.  */
+      if (p[len] == '\0')
+	return;
+
+      /* We did not find a valid name-value pair before encountering the
+	 colon.  */
+      if (p[len]== ':')
+	{
+	  p += len + 1;
+	  continue;
+	}
+
+      p += len + 1;
+
+      char *value = p;
+      len = 0;
+
+      while (p[len] != ':' && p[len] != '\0')
+	len++;
+
+      char end = p[len];
+      p[len] = '\0';
+
+      /* Add the tunable if it exists.  */
+      for (size_t i = 0; i < sizeof (tunable_list) / sizeof (tunable_t); i++)
+	{
+	  tunable_t *cur = &tunable_list[i];
+
+	  /* If we are in a secure context (AT_SECURE) then ignore the tunable
+	     unless it is explicitly marked as secure.  Tunable values take
+	     precendence over their envvar aliases.  */
+	  if (__libc_enable_secure && !cur->is_secure)
+	    continue;
+
+	  if (is_name (cur->name, name))
+	    {
+	      tunable_initialize (cur, value);
+	      break;
+	    }
+	}
+
+      if (end == ':')
+	p += len + 1;
+      else
+	return;
+    }
+}
+
+static size_t
+min_strlen (const char *s)
+{
+  size_t i = 0;
+  while (*s++ != '\0')
+    i++;
+
+  return i;
+}
+
 /* Disable a tunable if it is set.  */
 static void
 disable_tunable (tunable_id_t id, char **envp)
@@ -226,6 +328,23 @@ disable_tunable (tunable_id_t id, char **envp)
 
   if (env_alias != NULL)
     tunables_unsetenv (envp, tunable_list[id].env_alias);
+
+  char *tunable = getenv (GLIBC_TUNABLES);
+  const char *cmp = tunable_list[id].name;
+  const size_t len = min_strlen (cmp);
+
+  while (tunable && *tunable != '\0' && *tunable != ':')
+    {
+      if (is_name (tunable, cmp))
+	{
+	  tunable += len;
+	  /* Overwrite the = and the value with colons.  */
+	  while (*tunable != '\0' && *tunable != ':')
+	    *tunable++ = ':';
+	  break;
+	}
+      tunable++;
+    }
 }
 
 /* Disable the glibc.malloc.check tunable in SETUID/SETGID programs unless
@@ -256,6 +375,14 @@ __tunables_init (char **envp)
 
   while ((envp = get_next_env (envp, &envname, &len, &envval)) != NULL)
     {
+      if (is_name (GLIBC_TUNABLES, envname))
+	{
+	  char *val = tunables_strdup (envval);
+	  if (val != NULL)
+	    parse_tunables (val);
+	  continue;
+	}
+
       for (int i = 0; i < sizeof (tunable_list) / sizeof (tunable_t); i++)
 	{
 	  tunable_t *cur = &tunable_list[i];