[glibc/maskray/lld] elf: Avoid RELATIVE relocs in __tunables_init

[glibc/maskray/lld] elf: Avoid RELATIVE relocs in __tunables_init

[Fangrui Song]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=bfe04789a8d9014cf3177137651f8f93dd4a00ca

commit bfe04789a8d9014cf3177137651f8f93dd4a00ca
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Tue Jan 12 16:28:27 2021 +0000

    elf: Avoid RELATIVE relocs in __tunables_init
    
    With static pie linking pointers in the tunables list need
    RELATIVE relocs since the absolute address is not known at link
    time. We want to avoid relocations so the static pie self
    relocation can be done after tunables are initialized.
    
    This is a simple fix that embeds the tunable strings into the
    tunable list instead of using pointers.  It is possible to have
    a more compact representation of tunables with some additional
    complexity in the generator and tunable parser logic.  Such
    optimization will be useful if the list of tunables grows.
    
    There is still an issue that tunables_strdup allocates and the
    failure handling code path is sufficiently complex that it can
    easily have RELATIVE relocations.  It is possible to avoid the
    early allocation and only change environment variables in a
    setuid exe after relocations are processed.  But that is a
    bigger change and early failure is fatal anyway so it is not
    as critical to fix right away. This is bug 27181.
    
    Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

Diff:
---
 elf/dl-tunable-types.h   |  4 ++--
 elf/dl-tunables.c        |  2 +-
 scripts/gen-tunables.awk | 12 +++++++++++-
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/elf/dl-tunable-types.h b/elf/dl-tunable-types.h
index 05d4958e1c..3fcc0806f5 100644
--- a/elf/dl-tunable-types.h
+++ b/elf/dl-tunable-types.h
@@ -59,7 +59,7 @@ typedef enum
 /* A tunable.  */
 struct _tunable
 {
-  const char *name;			/* Internal name of the tunable.  */
+  const char name[TUNABLE_NAME_MAX];	/* Internal name of the tunable.  */
   tunable_type_t type;			/* Data type of the tunable.  */
   tunable_val_t val;			/* The value.  */
   bool initialized;			/* Flag to indicate that the tunable is
@@ -75,7 +75,7 @@ struct _tunable
 					   target module if the value is
 					   considered unsafe.  */
   /* Compatibility elements.  */
-  const char *env_alias;		/* The compatibility environment
+  const char env_alias[TUNABLE_ALIAS_MAX]; /* The compatibility environment
 					   variable name.  */
 };
 
diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index 33be00e447..e44476f204 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -351,7 +351,7 @@ __tunables_init (char **envp)
 
 	  /* Skip over tunables that have either been set already or should be
 	     skipped.  */
-	  if (cur->initialized || cur->env_alias == NULL)
+	  if (cur->initialized || cur->env_alias[0] == '\0')
 	    continue;
 
 	  const char *name = cur->env_alias;
diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
index cda12ef62e..fa63e86d1a 100644
--- a/scripts/gen-tunables.awk
+++ b/scripts/gen-tunables.awk
@@ -12,6 +12,8 @@ BEGIN {
   tunable=""
   ns=""
   top_ns=""
+  max_name_len=0
+  max_alias_len=0
 }
 
 # Skip over blank lines and comments.
@@ -57,11 +59,14 @@ $1 == "}" {
       maxvals[top_ns,ns,tunable] = max_of[types[top_ns,ns,tunable]]
     }
     if (!env_alias[top_ns,ns,tunable]) {
-      env_alias[top_ns,ns,tunable] = "NULL"
+      env_alias[top_ns,ns,tunable] = "{0}"
     }
     if (!security_level[top_ns,ns,tunable]) {
       security_level[top_ns,ns,tunable] = "SXID_ERASE"
     }
+    len = length(top_ns"."ns"."tunable)
+    if (len > max_name_len)
+      max_name_len = len
 
     tunable = ""
   }
@@ -109,6 +114,9 @@ $1 == "}" {
   }
   else if (attr == "env_alias") {
     env_alias[top_ns,ns,tunable] = sprintf("\"%s\"", val)
+    len = length(val)
+    if (len > max_alias_len)
+      max_alias_len = len
   }
   else if (attr == "security_level") {
     if (val == "SXID_ERASE" || val == "SXID_IGNORE" || val == "NONE") {
@@ -158,6 +166,8 @@ END {
 
   print "\n#ifdef TUNABLES_INTERNAL"
   # Internal definitions.
+  print "# define TUNABLE_NAME_MAX " (max_name_len + 1)
+  print "# define TUNABLE_ALIAS_MAX " (max_alias_len + 1)
   print "# include \"dl-tunable-types.h\""
   # Finally, the tunable list.
   print "static tunable_t tunable_list[] attribute_relro = {"