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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>
This commit is contained in:
Szabolcs Nagy 2021-01-12 16:28:27 +00:00
parent 058308066c
commit bfe04789a8
3 changed files with 14 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
elf/dl-tunable-types.h
View File

@ -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. */
};

2
elf/dl-tunables.c
View File

@ -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;

12
scripts/gen-tunables.awk
View File

@ -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 = {"