Re: [PATCH 1/4] kallsyms: add support for relative offsets in kallsyms address table

From: Kees Cook
Date: Wed Jan 20 2016 - 14:13:28 EST

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On Wed, Jan 20, 2016 at 1:05 AM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> 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. The benefit
> is that such table entries are no longer subject to dynamic relocation when
> the build time and runtime offsets of the kernel image are different. Also,
> on 64-bit architectures, it essentially cuts the size of the address table
> in half since offsets can typically be expressed in 32 bits.
>
> Since it is useful for some architectures (like x86) to retain the ability
> to emit absolute values as well, this patch adds support for both, by
> emitting absolute addresses as positive 32-bit values, and addresses
> relative to _text as negative values, which are subtracted from the runtime
> address of _text to produce the actual address. Positive values are used as
> they are found in the table.
>
> Support for the above is enabled by setting CONFIG_KALLSYMS_TEXT_RELATIVE.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

A nice space-saver! :)

-Kees

> ---
> init/Kconfig | 14 ++++++++
> kernel/kallsyms.c | 35 +++++++++++++-----
> scripts/kallsyms.c | 38 +++++++++++++++++---
> scripts/link-vmlinux.sh | 4 +++
> scripts/namespace.pl | 1 +
> 5 files changed, 79 insertions(+), 13 deletions(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 5b86082fa238..73e00b040572 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1427,6 +1427,20 @@ config KALLSYMS_ALL
>
> Say N unless you really need all symbols.
>
> +config KALLSYMS_TEXT_RELATIVE
> + bool
> + 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 either an absolute value in the range [0, S32_MAX] or
> + a text relative value in the range [_text, _text + S32_MAX], encoded
> + as negative values.
> +
> + 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..e612f7f9e71b 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;
>
> /*
> @@ -176,6 +177,19 @@ 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_TEXT_RELATIVE))
> + return kallsyms_addresses[idx];
> +
> + /* positive offsets are absolute values */
> + if (kallsyms_offsets[idx] >= 0)
> + return kallsyms_offsets[idx];
> +
> + /* negative offsets are relative to _text - 1 */
> + return (unsigned long)_text - 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 +201,7 @@ 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 +218,7 @@ 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 +234,10 @@ 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_TEXT_RELATIVE))
> + BUG_ON(!kallsyms_addresses);
> + else
> + BUG_ON(!kallsyms_offsets);
>
> /* Do a binary search on the sorted kallsyms_addresses array. */
> low = 0;
> @@ -228,7 +245,7 @@ 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 +255,15 @@ 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 +487,7 @@ 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 8fa81e84e295..07656c102e60 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]))
> @@ -61,6 +62,7 @@ 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 text_relative = 0;
>
> int token_profit[0x10000];
>
> @@ -74,7 +76,7 @@ static void usage(void)
> fprintf(stderr, "Usage: kallsyms [--all-symbols] "
> "[--symbol-prefix=<prefix char>] "
> "[--page-offset=<CONFIG_PAGE_OFFSET>] "
> - "< in.map > out.S\n");
> + "[--text-relative] < in.map > out.S\n");
> exit(1);
> }
>
> @@ -202,6 +204,7 @@ static int symbol_valid(struct sym_entry *s)
> */
> static char *special_symbols[] = {
> "kallsyms_addresses",
> + "kallsyms_offsets",
> "kallsyms_num_syms",
> "kallsyms_names",
> "kallsyms_markers",
> @@ -353,9 +356,34 @@ static void write_src(void)
> * .o files. This prevents .tmp_kallsyms.o or any other
> * object from referencing them.
> */
> - output_label("kallsyms_addresses");
> + if (!text_relative)
> + output_label("kallsyms_addresses");
> + else
> + output_label("kallsyms_offsets");
> +
> for (i = 0; i < table_cnt; i++) {
> - if (!symbol_absolute(&table[i])) {
> + if (text_relative) {
> + long long offset;
> +
> + if (symbol_absolute(&table[i])) {
> + offset = table[i].addr;
> + if (offset < 0 || offset > INT_MAX) {
> + fprintf(stderr, "kallsyms failure: "
> + "absolute symbol value %#llx out of range in relative mode\n",
> + table[i].addr);
> + exit(EXIT_FAILURE);
> + }
> + } else {
> + offset = _text - table[i].addr - 1;
> + if (offset < INT_MIN || offset >= 0) {
> + fprintf(stderr, "kallsyms failure: "
> + "relative symbol value %#llx out of range in relative mode\n",
> + 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);
> @@ -703,7 +731,9 @@ 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], "--text-relative") == 0)
> + text_relative = 1;
> + else
> usage();
> }
> } else if (argc != 1)
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index ba6c34ea5429..e0f957f6a54c 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_TEXT_RELATIVE}" ]; then
> + kallsymopt="${kallsymopt} --text-relative"
> + fi
> +
> local aflags="${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL} \
> ${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS}"
>
> diff --git a/scripts/namespace.pl b/scripts/namespace.pl
> index a71be6b7cdec..e059ab240364 100755
> --- a/scripts/namespace.pl
> +++ b/scripts/namespace.pl
> @@ -117,6 +117,7 @@ my %nameexception = (
> 'kallsyms_names' => 1,
> 'kallsyms_num_syms' => 1,
> 'kallsyms_addresses'=> 1,
> + 'kallsyms_offsets' => 1,
> '__this_module' => 1,
> '_etext' => 1,
> '_edata' => 1,
> --
> 2.5.0
>

--
Kees Cook
Chrome OS & Brillo Security

Next message: Michael S. Tsirkin: "[PATCH] tools/virtio: use virt_xxx barriers"
Previous message: Kees Cook: "Re: [PATCH 4/4] x86_64: enable text relative kallsyms for 64-bit targets"
In reply to: Ard Biesheuvel: "[PATCH 1/4] kallsyms: add support for relative offsets in kallsyms address table"
Next in thread: Ard Biesheuvel: "[PATCH 2/4] powerpc: enable text relative kallsyms for ppc64"
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