* [PATCH v5 2/3] kallsyms: don't overload absolute symbol type for percpu symbols
2016-02-08 17:57 [PATCH v5 0/3] kallsyms base relative series Ard Biesheuvel
2016-02-08 17:57 ` [PATCH v5 1/3] x86: kallsyms: disable absolute percpu symbols on !SMP Ard Biesheuvel
@ 2016-02-08 17:57 ` Ard Biesheuvel
2016-02-08 17:57 ` [PATCH v5 3/3] kallsyms: add support for relative offsets in kallsyms address table Ard Biesheuvel
2 siblings, 0 replies; 4+ messages in thread
From: Ard Biesheuvel @ 2016-02-08 17:57 UTC (permalink / raw)
To: linux-kernel, linux-s390, linuxppc-dev, x86, keescook, akpm,
mingo, hpa, heiko.carstens, benh, mpe, mmarek, rusty, arnd,
linux-arch, linux
Cc: Ard Biesheuvel
Commit c6bda7c988a5 ("kallsyms: fix percpu vars on x86-64 with relocation")
overloaded the 'A' (absolute) symbol type to signify that a symbol is not
subject to dynamic relocation. However, the original A type does not imply
that at all, and depending on the version of the toolchain, many A type
symbols are emitted that are in fact relative to the kernel text, i.e., if
the kernel is relocated at runtime, these symbols should be updated as well.
For instance, on sparc32, the following symbols are emitted as absolute
(kindly provided by Guenter Roeck):
f035a420 A _etext
f03d9000 A _sdata
f03de8c4 A jiffies
f03f8860 A _edata
f03fc000 A __init_begin
f041bdc8 A __init_text_end
f0423000 A __bss_start
f0423000 A __init_end
f044457d A __bss_stop
f044457d A _end
On x86_64, similar behavior can be observed:
ffffffff81a00000 A __end_rodata_hpage_align
ffffffff81b19000 A __vvar_page
ffffffff81d3d000 A _end
Even if only a couple of them pass the symbol range check that results in
them to be taken into account for the final kallsyms symbol table, it is
obvious that 'A' does not mean the symbol does not need to be updated at
relocation time, and overloading its meaning to signify that is perhaps not
a good idea.
So instead, add a new percpu_absolute member to struct sym_entry, and when
--absolute-percpu is in effect, use it to record symbols whose addresses
should be emitted as final values rather than values that still require
relocation at runtime. That way, we can drop the check against the 'A' type.
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
scripts/kallsyms.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 8fa81e84e295..d39a1eeb080e 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -34,6 +34,7 @@ struct sym_entry {
unsigned int len;
unsigned int start_pos;
unsigned char *sym;
+ unsigned int percpu_absolute;
};
struct addr_range {
@@ -171,6 +172,8 @@ static int read_symbol(FILE *in, struct sym_entry *s)
strcpy((char *)s->sym + 1, str);
s->sym[0] = stype;
+ s->percpu_absolute = 0;
+
/* Record if we've found __per_cpu_start/end. */
check_symbol_range(sym, s->addr, &percpu_range, 1);
@@ -325,7 +328,7 @@ static int expand_symbol(unsigned char *data, int len, char *result)
static int symbol_absolute(struct sym_entry *s)
{
- return toupper(s->sym[0]) == 'A';
+ return s->percpu_absolute;
}
static void write_src(void)
@@ -681,8 +684,15 @@ static void make_percpus_absolute(void)
unsigned int i;
for (i = 0; i < table_cnt; i++)
- if (symbol_in_range(&table[i], &percpu_range, 1))
+ if (symbol_in_range(&table[i], &percpu_range, 1)) {
+ /*
+ * Keep the 'A' override for percpu symbols to
+ * ensure consistent behavior compared to older
+ * versions of this tool.
+ */
table[i].sym[0] = 'A';
+ table[i].percpu_absolute = 1;
+ }
}
int main(int argc, char **argv)
--
2.5.0
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v5 3/3] kallsyms: add support for relative offsets in kallsyms address table
2016-02-08 17:57 [PATCH v5 0/3] kallsyms base relative series Ard Biesheuvel
2016-02-08 17:57 ` [PATCH v5 1/3] x86: kallsyms: disable absolute percpu symbols on !SMP Ard Biesheuvel
2016-02-08 17:57 ` [PATCH v5 2/3] kallsyms: don't overload absolute symbol type for percpu symbols Ard Biesheuvel
@ 2016-02-08 17:57 ` Ard Biesheuvel
2 siblings, 0 replies; 4+ messages in thread
From: Ard Biesheuvel @ 2016-02-08 17:57 UTC (permalink / raw)
To: linux-kernel, linux-s390, linuxppc-dev, x86, keescook, akpm,
mingo, hpa, heiko.carstens, benh, mpe, mmarek, rusty, arnd,
linux-arch, linux
Cc: Ard Biesheuvel
Similar to how relative extables are implemented, it is possible to emit
the kallsyms table in such a way that it contains offsets relative to some
anchor point in the kernel image rather than absolute addresses.
On 64-bit architectures, it cuts the size of the kallsyms address table in
half, since offsets between kernel symbols can typically be expressed in 32
bits. This saves several hundreds of kilobytes of permanent .rodata on
average. In addition, the kallsyms address table is no longer subject to
dynamic relocation when CONFIG_RELOCATABLE is in effect, so the relocation
work done after decompression now doesn't have to do relocation updates for
all these values. This saves up to 24 bytes (i.e., the size of a ELF64 RELA
relocation table entry) per value, which easily adds up to a couple of
megabytes of uncompressed __init data on ppc64 or arm64. Even if these
relocation entries typically compress well, the combined size reduction of
2.8 MB uncompressed for a ppc64_defconfig build (of which 2.4 MB is __init
data) results in a ~500 KB space saving in the compressed image.
Since it is useful for some architectures (like x86) to retain the ability
to emit absolute values as well, this patch also adds support for capturing
both absolute and relative values when KALLSYMS_ABSOLUTE_PERCPU is in
effect, by emitting absolute per-cpu addresses as positive 32-bit values,
and addresses relative to the lowest encountered relative symbol as
negative values, which are subtracted from the runtime address of this base
symbol to produce the actual address.
Support for the above is enabled by default for all architectures except
IA-64 and Tile-GX, whose symbols are too far apart to capture in this
manner.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
init/Kconfig | 18 ++++
kernel/kallsyms.c | 42 +++++++--
scripts/kallsyms.c | 89 +++++++++++++++++---
scripts/link-vmlinux.sh | 4 +
scripts/namespace.pl | 2 +
5 files changed, 136 insertions(+), 19 deletions(-)
diff --git a/init/Kconfig b/init/Kconfig
index b17824a875fa..6d48682fbcae 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1424,6 +1424,24 @@ config KALLSYMS_ABSOLUTE_PERCPU
bool
default X86_64 && SMP
+config KALLSYMS_BASE_RELATIVE
+ bool
+ depends on KALLSYMS
+ default !IA64 && !(TILE && 64BIT)
+ help
+ Instead of emitting them as absolute values in the native word size,
+ emit the symbol references in the kallsyms table as 32-bit entries,
+ each containing a relative value in the range [base, base + U32_MAX]
+ or, when KALLSYMS_ABSOLUTE_PERCPU is in effect, each containing either
+ an absolute value in the range [0, S32_MAX] or a relative value in the
+ range [base, base + S32_MAX], where base is the lowest relative symbol
+ address encountered in the image.
+
+ On 64-bit builds, this reduces the size of the address table by 50%,
+ but more importantly, it results in entries whose values are build
+ time constants, and no relocation pass is required at runtime to fix
+ up the entries based on the runtime load address of the kernel.
+
config PRINTK
default y
bool "Enable support for printk" if EXPERT
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 5c5987f10819..fafd1a3ef0da 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -38,6 +38,7 @@
* during the second link stage.
*/
extern const unsigned long kallsyms_addresses[] __weak;
+extern const int kallsyms_offsets[] __weak;
extern const u8 kallsyms_names[] __weak;
/*
@@ -47,6 +48,9 @@ extern const u8 kallsyms_names[] __weak;
extern const unsigned long kallsyms_num_syms
__attribute__((weak, section(".rodata")));
+extern const unsigned long kallsyms_relative_base
+__attribute__((weak, section(".rodata")));
+
extern const u8 kallsyms_token_table[] __weak;
extern const u16 kallsyms_token_index[] __weak;
@@ -176,6 +180,23 @@ static unsigned int get_symbol_offset(unsigned long pos)
return name - kallsyms_names;
}
+static unsigned long kallsyms_sym_address(int idx)
+{
+ if (!IS_ENABLED(CONFIG_KALLSYMS_BASE_RELATIVE))
+ return kallsyms_addresses[idx];
+
+ /* values are unsigned offsets if --absolute-percpu is not in effect */
+ if (!IS_ENABLED(CONFIG_KALLSYMS_ABSOLUTE_PERCPU))
+ return kallsyms_relative_base + (u32)kallsyms_offsets[idx];
+
+ /* ...otherwise, positive offsets are absolute values */
+ if (kallsyms_offsets[idx] >= 0)
+ return kallsyms_offsets[idx];
+
+ /* ...and negative offsets are relative to kallsyms_relative_base - 1 */
+ return kallsyms_relative_base - 1 - kallsyms_offsets[idx];
+}
+
/* Lookup the address for this symbol. Returns 0 if not found. */
unsigned long kallsyms_lookup_name(const char *name)
{
@@ -187,7 +208,7 @@ unsigned long kallsyms_lookup_name(const char *name)
off = kallsyms_expand_symbol(off, namebuf, ARRAY_SIZE(namebuf));
if (strcmp(namebuf, name) == 0)
- return kallsyms_addresses[i];
+ return kallsyms_sym_address(i);
}
return module_kallsyms_lookup_name(name);
}
@@ -204,7 +225,7 @@ int kallsyms_on_each_symbol(int (*fn)(void *, const char *, struct module *,
for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
off = kallsyms_expand_symbol(off, namebuf, ARRAY_SIZE(namebuf));
- ret = fn(data, namebuf, NULL, kallsyms_addresses[i]);
+ ret = fn(data, namebuf, NULL, kallsyms_sym_address(i));
if (ret != 0)
return ret;
}
@@ -220,7 +241,10 @@ static unsigned long get_symbol_pos(unsigned long addr,
unsigned long i, low, high, mid;
/* This kernel should never had been booted. */
- BUG_ON(!kallsyms_addresses);
+ if (!IS_ENABLED(CONFIG_KALLSYMS_BASE_RELATIVE))
+ BUG_ON(!kallsyms_addresses);
+ else
+ BUG_ON(!kallsyms_offsets);
/* Do a binary search on the sorted kallsyms_addresses array. */
low = 0;
@@ -228,7 +252,7 @@ static unsigned long get_symbol_pos(unsigned long addr,
while (high - low > 1) {
mid = low + (high - low) / 2;
- if (kallsyms_addresses[mid] <= addr)
+ if (kallsyms_sym_address(mid) <= addr)
low = mid;
else
high = mid;
@@ -238,15 +262,15 @@ static unsigned long get_symbol_pos(unsigned long addr,
* Search for the first aliased symbol. Aliased
* symbols are symbols with the same address.
*/
- while (low && kallsyms_addresses[low-1] == kallsyms_addresses[low])
+ while (low && kallsyms_sym_address(low-1) == kallsyms_sym_address(low))
--low;
- symbol_start = kallsyms_addresses[low];
+ symbol_start = kallsyms_sym_address(low);
/* Search for next non-aliased symbol. */
for (i = low + 1; i < kallsyms_num_syms; i++) {
- if (kallsyms_addresses[i] > symbol_start) {
- symbol_end = kallsyms_addresses[i];
+ if (kallsyms_sym_address(i) > symbol_start) {
+ symbol_end = kallsyms_sym_address(i);
break;
}
}
@@ -470,7 +494,7 @@ static unsigned long get_ksymbol_core(struct kallsym_iter *iter)
unsigned off = iter->nameoff;
iter->module_name[0] = '\0';
- iter->value = kallsyms_addresses[iter->pos];
+ iter->value = kallsyms_sym_address(iter->pos);
iter->type = kallsyms_get_symbol_type(off);
diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index d39a1eeb080e..073d4601b982 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -22,6 +22,7 @@
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
+#include <limits.h>
#ifndef ARRAY_SIZE
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof(arr[0]))
@@ -43,6 +44,7 @@ struct addr_range {
};
static unsigned long long _text;
+static unsigned long long relative_base;
static struct addr_range text_ranges[] = {
{ "_stext", "_etext" },
{ "_sinittext", "_einittext" },
@@ -62,6 +64,7 @@ static int all_symbols = 0;
static int absolute_percpu = 0;
static char symbol_prefix_char = '\0';
static unsigned long long kernel_start_addr = 0;
+static int base_relative = 0;
int token_profit[0x10000];
@@ -75,7 +78,7 @@ static void usage(void)
fprintf(stderr, "Usage: kallsyms [--all-symbols] "
"[--symbol-prefix=<prefix char>] "
"[--page-offset=<CONFIG_PAGE_OFFSET>] "
- "< in.map > out.S\n");
+ "[--base-relative] < in.map > out.S\n");
exit(1);
}
@@ -205,6 +208,8 @@ static int symbol_valid(struct sym_entry *s)
*/
static char *special_symbols[] = {
"kallsyms_addresses",
+ "kallsyms_offsets",
+ "kallsyms_relative_base",
"kallsyms_num_syms",
"kallsyms_names",
"kallsyms_markers",
@@ -349,16 +354,48 @@ static void write_src(void)
printf("\t.section .rodata, \"a\"\n");
- /* Provide proper symbols relocatability by their '_text'
- * relativeness. The symbol names cannot be used to construct
- * normal symbol references as the list of symbols contains
- * symbols that are declared static and are private to their
- * .o files. This prevents .tmp_kallsyms.o or any other
- * object from referencing them.
+ /* Provide proper symbols relocatability by their relativeness
+ * to a fixed anchor point in the runtime image, either '_text'
+ * for absolute address tables, in which case the linker will
+ * emit the final addresses at build time. Otherwise, use the
+ * offset relative to the lowest value encountered of all relative
+ * symbols, and emit non-relocatable fixed offsets that will be fixed
+ * up at runtime.
+ *
+ * The symbol names cannot be used to construct normal symbol
+ * references as the list of symbols contains symbols that are
+ * declared static and are private to their .o files. This prevents
+ * .tmp_kallsyms.o or any other object from referencing them.
*/
- output_label("kallsyms_addresses");
+ if (!base_relative)
+ output_label("kallsyms_addresses");
+ else
+ output_label("kallsyms_offsets");
+
for (i = 0; i < table_cnt; i++) {
- if (!symbol_absolute(&table[i])) {
+ if (base_relative) {
+ long long offset;
+ int overflow;
+
+ if (!absolute_percpu) {
+ offset = table[i].addr - relative_base;
+ overflow = (offset < 0 || offset > UINT_MAX);
+ } else if (symbol_absolute(&table[i])) {
+ offset = table[i].addr;
+ overflow = (offset < 0 || offset > INT_MAX);
+ } else {
+ offset = relative_base - table[i].addr - 1;
+ overflow = (offset < INT_MIN || offset >= 0);
+ }
+ if (overflow) {
+ fprintf(stderr, "kallsyms failure: "
+ "%s symbol value %#llx out of range in relative mode\n",
+ symbol_absolute(&table[i]) ? "absolute" : "relative",
+ table[i].addr);
+ exit(EXIT_FAILURE);
+ }
+ printf("\t.long\t%#x\n", (int)offset);
+ } else if (!symbol_absolute(&table[i])) {
if (_text <= table[i].addr)
printf("\tPTR\t_text + %#llx\n",
table[i].addr - _text);
@@ -371,6 +408,12 @@ static void write_src(void)
}
printf("\n");
+ if (base_relative) {
+ output_label("kallsyms_relative_base");
+ printf("\tPTR\t_text - %#llx\n", _text - relative_base);
+ printf("\n");
+ }
+
output_label("kallsyms_num_syms");
printf("\tPTR\t%d\n", table_cnt);
printf("\n");
@@ -695,6 +738,18 @@ static void make_percpus_absolute(void)
}
}
+/* find the minimum non-absolute symbol address */
+static void record_relative_base(void)
+{
+ unsigned int i;
+
+ relative_base = -1ULL;
+ for (i = 0; i < table_cnt; i++)
+ if (!symbol_absolute(&table[i]) &&
+ table[i].addr < relative_base)
+ relative_base = table[i].addr;
+}
+
int main(int argc, char **argv)
{
if (argc >= 2) {
@@ -713,7 +768,9 @@ int main(int argc, char **argv)
} else if (strncmp(argv[i], "--page-offset=", 14) == 0) {
const char *p = &argv[i][14];
kernel_start_addr = strtoull(p, NULL, 16);
- } else
+ } else if (strcmp(argv[i], "--base-relative") == 0)
+ base_relative = 1;
+ else
usage();
}
} else if (argc != 1)
@@ -722,6 +779,8 @@ int main(int argc, char **argv)
read_map(stdin);
if (absolute_percpu)
make_percpus_absolute();
+ if (base_relative)
+ record_relative_base();
sort_symbols();
optimize_token_table();
write_src();
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 7a08bf9a9576..453ede9d2f3d 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -90,6 +90,10 @@ kallsyms()
kallsymopt="${kallsymopt} --absolute-percpu"
fi
+ if [ -n "${CONFIG_KALLSYMS_BASE_RELATIVE}" ]; then
+ kallsymopt="${kallsymopt} --base-relative"
+ fi
+
local aflags="${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL} \
${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS}"
diff --git a/scripts/namespace.pl b/scripts/namespace.pl
index a71be6b7cdec..9f3c9d47a4a5 100755
--- a/scripts/namespace.pl
+++ b/scripts/namespace.pl
@@ -117,6 +117,8 @@ my %nameexception = (
'kallsyms_names' => 1,
'kallsyms_num_syms' => 1,
'kallsyms_addresses'=> 1,
+ 'kallsyms_offsets' => 1,
+ 'kallsyms_relative_base'=> 1,
'__this_module' => 1,
'_etext' => 1,
'_edata' => 1,
--
2.5.0
^ permalink raw reply related [flat|nested] 4+ messages in thread