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Message-ID: <CAK7LNAQZO0g-B7UUEvdJWh3FhdhmWaaSaJyyEUoVoSYG0j8v-Q@mail.gmail.com>
Date: Thu, 21 Dec 2023 22:40:33 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: deller@...nel.org
Cc: linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
linux-modules@...r.kernel.org, linux-arch@...r.kernel.org,
Luis Chamberlain <mcgrof@...nel.org>
Subject: Re: [PATCH 0/4] Section alignment issues?
On Thu, Nov 23, 2023 at 7:18 AM <deller@...nel.org> wrote:
>
> From: Helge Deller <deller@....de>
>
> While working on the 64-bit parisc kernel, I noticed that the __ksymtab[]
> table was not correctly 64-bit aligned in many modules.
> The following patches do fix some of those issues in the generic code.
>
> But further investigation shows that multiple sections in the kernel and in
> modules are possibly not correctly aligned, and thus may lead to performance
> degregations at runtime (small on x86, huge on parisc, sparc and others which
> need exception handlers). Sometimes wrong alignments may also be simply hidden
> by the linker or kernel module loader which pulls in the sections by luck with
> a correct alignment (e.g. because the previous section was aligned already).
>
> An objdump on a x86 module shows e.g.:
>
> ./kernel/net/netfilter/nf_log_syslog.ko: file format elf64-x86-64
> Sections:
> Idx Name Size VMA LMA File off Algn
> 0 .text 00001fdf 0000000000000000 0000000000000000 00000040 2**4
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
> 1 .init.text 000000f6 0000000000000000 0000000000000000 00002020 2**4
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
> 2 .exit.text 0000005c 0000000000000000 0000000000000000 00002120 2**4
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
> 3 .rodata.str1.8 000000dc 0000000000000000 0000000000000000 00002180 2**3
> CONTENTS, ALLOC, LOAD, READONLY, DATA
> 4 .rodata.str1.1 0000030a 0000000000000000 0000000000000000 0000225c 2**0
> CONTENTS, ALLOC, LOAD, READONLY, DATA
> 5 .rodata 000000b0 0000000000000000 0000000000000000 00002580 2**5
> CONTENTS, ALLOC, LOAD, READONLY, DATA
> 6 .modinfo 0000019e 0000000000000000 0000000000000000 00002630 2**0
> CONTENTS, ALLOC, LOAD, READONLY, DATA
> 7 .return_sites 00000034 0000000000000000 0000000000000000 000027ce 2**0
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
> 8 .call_sites 0000029c 0000000000000000 0000000000000000 00002802 2**0
> CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
>
> In this example I believe the ".return_sites" and ".call_sites" should have
> an alignment of at least 32-bit (4 bytes).
>
> On other architectures or modules other sections like ".altinstructions" or
> "__ex_table" may show up wrongly instead.
>
> In general I think it would be beneficial to search for wrong alignments at
> link time, and maybe at runtime.
>
> The patch at the end of this cover letter
> - adds compile time checks to the "modpost" tool, and
> - adds a runtime check to the kernel module loader at runtime.
> And it will possibly show false positives too (!!!)
> I do understand that some of those sections are not performce critical
> and thus any alignment is OK.
>
> The modpost patch will emit at compile time such warnings (on x86-64 kernel build):
>
> WARNING: modpost: vmlinux: section .initcall7.init (type 1, flags 2) has alignment of 1, expected at least 4.
> Maybe you need to add ALIGN() to the modules.lds file (or fix modpost) ?
> WARNING: modpost: vmlinux: section .altinstructions (type 1, flags 2) has alignment of 1, expected at least 2.
> WARNING: modpost: vmlinux: section .initcall6.init (type 1, flags 2) has alignment of 1, expected at least 4.
> WARNING: modpost: vmlinux: section .initcallearly.init (type 1, flags 2) has alignment of 1, expected at least 4.
> WARNING: modpost: vmlinux: section .rodata.cst2 (type 1, flags 18) has alignment of 2, expected at least 64.
> WARNING: modpost: vmlinux: section .static_call_tramp_key (type 1, flags 2) has alignment of 1, expected at least 8.
> WARNING: modpost: vmlinux: section .con_initcall.init (type 1, flags 2) has alignment of 1, expected at least 8.
> WARNING: modpost: vmlinux: section __bug_table (type 1, flags 3) has alignment of 1, expected at least 4.
> ...
modpost acts on vmlinux.o instead of vmlinux.
vmlinux.o is a relocatable ELF, which is not a real layout
because no linker script has been considered yet at this
point.
vmlinux is an executable ELF, produced by a linker,
with the linker script taken into consideration
to determine the final section/symbol layout.
So, checking this in modpost is meaningless.
I did not check the module checking code, but
modules are also relocatable ELF.
>
> while the kernel module loader may show at runtime:
>
> ** WARNING **: module: efivarfs, dest=0xffffffffc02d08d2, section=.retpoline_sites possibly not correctly aligned.
> ** WARNING **: module: efivarfs, dest=0xffffffffc02d0bae, section=.ibt_endbr_seal possibly not correctly aligned.
> ** WARNING **: module: efivarfs, dest=0xffffffffc02d0be6, section=.orc_unwind possibly not correctly aligned.
> ** WARNING **: module: efivarfs, dest=0xffffffffc02d12a6, section=.orc_unwind_ip possibly not correctly aligned.
> ** WARNING **: module: efivarfs, dest=0xffffffffc02d178c, section=.note.Linux possibly not correctly aligned.
> ** WARNING **: module: efivarfs, dest=0xffffffffc02d17bc, section=.orc_header possibly not correctly aligned.
> ** WARNING **: module: xt_addrtype, dest=0xffffffffc01ef18a, size=0x00000020, section=.return_sites
>
> My questions:
> - Am I wrong with my analysis?
> - What does people see on other architectures?
> - Does it make sense to add a compile- and runtime-check, like the patch below, to the kernel?
>
> Helge
>
> ---
>
> From: Helge Deller <deller@....de>
> Subject: [PATCH] MODPOST: Detect and report possible section alignment issues
>
> IMPORTANT: THIS PATCH IS NOT INTENDED TO BE APPLIED !!!!
>
> The main idea here is to check at compile time (during modpost run) and at
> runtime (when loading a module) if the sections in kernel modules (and
> vmlinux) are correctly aligned in memory and report if a wrong alignment is
> suspected.
>
> It WILL report false positives. Many section names still need to be added to
> the various tables.
> But even at this stage it gives some insight at the various possibly wrong
> section alignments.
>
> Signed-off-by: Helge Deller <deller@....de>
>
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 98fedfdb8db5..88201d6b0c17 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -2277,6 +2277,10 @@ static int move_module(struct module *mod, struct load_info *info)
> ret = -ENOEXEC;
> goto out_enomem;
> }
> + if (((uintptr_t)dest) & (BITS_PER_LONG/8 - 1)) {
> + printk("** WARNING **: \tmodule: %s, dest=0x%lx, section=%s possibly not correctly aligned.\n",
> + mod->name, (long)dest, info->secstrings + shdr->sh_name);
> + }
> memcpy(dest, (void *)shdr->sh_addr, shdr->sh_size);
> }
> /*
> diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> index 739402f45509..2add144a05e3 100644
> --- a/scripts/Makefile.modpost
> +++ b/scripts/Makefile.modpost
> @@ -49,6 +49,8 @@ modpost-args = \
> $(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS)) \
> $(if $(CONFIG_MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS)$(KBUILD_NSDEPS),-N) \
> $(if $(findstring 1, $(KBUILD_EXTRA_WARN)),-W) \
> + $(if $(CONFIG_64BIT),-6) \
> + $(if $(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS),-R) \
> -o $@
>
> modpost-deps := $(MODPOST)
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index cb6406f485a9..70c69db6a17c 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -43,6 +43,10 @@ static bool ignore_missing_files;
> /* If set to 1, only warn (instead of error) about missing ns imports */
> static bool allow_missing_ns_imports;
>
> +/* is target a 64-bit platform and has it prel32 relocation support? */
> +static bool target_64bit;
> +static bool target_prel32_relocations;
> +
> static bool error_occurred;
>
> static bool extra_warn;
> @@ -1493,6 +1497,76 @@ static void check_sec_ref(struct module *mod, struct elf_info *elf)
> }
> }
>
> +/**
> + * Check alignment of sections in modules.
> + **/
> +static void check_sec_alignment(struct module *mod, struct elf_info *elf)
> +{
> + /* sections that may use PREL32 relocations and only need 4-byte alignment */
> + static const char *const prel32_sec_list[] = {
> + "__tracepoints_ptrs",
> + "__ksymtab",
> + "__bug_table",
> + ".smp_locks",
> + NULL
> + };
> + /* sections that are fine with any/1-byte alignment */
> + static const char *const byte_sec_list[] = {
> + ".modinfo",
> + ".init.ramfs",
> + NULL
> + };
> + /* sections with special alignment */
> + static struct { int align; const char *name; } const special_list[] = {
> + { 64, ".rodata.cst2" },
> + { 0, NULL }
> + };
> +
> + int i;
> +
> + /* ignore vmlinux for now? */
> + // if (mod->is_vmlinux) return;
> +
> + /* Walk through all sections */
> + for (i = 0; i < elf->num_sections; i++) {
> + Elf_Shdr *sechdr = &elf->sechdrs[i];
> + const char *sec = sech_name(elf, sechdr);
> + const char *modname = mod->name;
> + const int is_shalign = sechdr->sh_addralign;
> + int should_shalign;
> + int k;
> +
> + /* ignore some sections */
> + if ((sechdr->sh_type == SHT_NULL) ||
> + !(sechdr->sh_flags & SHF_ALLOC))
> + continue;
> +
> + /* default alignment is 8 for 64-bit and 4 for 32-bit targets * */
> + should_shalign = target_64bit ? 8 : 4;
> + if (match(sec, prel32_sec_list))
> + should_shalign = target_prel32_relocations ? 4 : should_shalign;
> + else if (strstr(sec, "text")) /* assume text segments to be 4-byte aligned */
> + should_shalign = 4;
> + else if (match(sec, byte_sec_list)) /* any alignment is OK. */
> + continue;
> + else {
> + /* search in list with special alignment */
> + k = 0;
> + while (special_list[k].align && strstarts(sec, special_list[k].name)) {
> + should_shalign = special_list[k].align;
> + break;
> + }
> + }
> +
> + if (is_shalign >= should_shalign)
> + continue;
> +
> + warn("%s: section %s (type %d, flags %lu) has alignment of %d, expected at least %d.\n"
> + "Maybe you need to add ALIGN() to the modules.lds file (or fix modpost) ?\n",
> + modname, sec, sechdr->sh_type, sechdr->sh_flags, is_shalign, should_shalign);
> + }
> +}
> +
> static char *remove_dot(char *s)
> {
> size_t n = strcspn(s, ".");
> @@ -1653,6 +1727,8 @@ static void read_symbols(const char *modname)
> handle_moddevtable(mod, &info, sym, symname);
> }
>
> + check_sec_alignment(mod, &info);
> +
> check_sec_ref(mod, &info);
>
> if (!mod->is_vmlinux) {
> @@ -2183,7 +2259,7 @@ int main(int argc, char **argv)
> LIST_HEAD(dump_lists);
> struct dump_list *dl, *dl2;
>
> - while ((opt = getopt(argc, argv, "ei:MmnT:to:au:WwENd:")) != -1) {
> + while ((opt = getopt(argc, argv, "ei:MmnT:to:au:WwENd:6R")) != -1) {
> switch (opt) {
> case 'e':
> external_module = true;
> @@ -2232,6 +2308,12 @@ int main(int argc, char **argv)
> case 'd':
> missing_namespace_deps = optarg;
> break;
> + case '6':
> + target_64bit = true;
> + break;
> + case 'R':
> + target_prel32_relocations = true;
> + break;
> default:
> exit(1);
> }
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 70c69db6a17c..b09ab081dc03 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1510,15 +1510,24 @@ static void check_sec_alignment(struct module *mod, struct elf_info *elf)
> ".smp_locks",
> NULL
> };
> - /* sections that are fine with any/1-byte alignment */
> + /* sections that are fine with any alignment */
> static const char *const byte_sec_list[] = {
> ".modinfo",
> ".init.ramfs",
> NULL
> };
> - /* sections with special alignment */
> + /* sections with special given minimal alignment. Checked with
> + * startswith(). */
> static struct { int align; const char *name; } const special_list[] = {
> { 64, ".rodata.cst2" },
> + { 2, ".altinstructions" }, /* at least on x86 */
> + { 1, ".altinstr_replacement" },
> + { 1, ".altinstr_aux" },
> + { 4, ".call_sites" },
> + { 4, ".return_sites" },
> + { 1, ".note.Linux" }, /* correct? */
> + { 4, "__ex_table" },
> + { 4, ".initcall" }, /* at least 4 ? */
> { 0, NULL }
> };
>
> @@ -1545,16 +1554,17 @@ static void check_sec_alignment(struct module *mod, struct elf_info *elf)
> should_shalign = target_64bit ? 8 : 4;
> if (match(sec, prel32_sec_list))
> should_shalign = target_prel32_relocations ? 4 : should_shalign;
> - else if (strstr(sec, "text")) /* assume text segments to be 4-byte aligned */
> - should_shalign = 4;
> else if (match(sec, byte_sec_list)) /* any alignment is OK. */
> continue;
> + else if (strstr(sec, "text")) /* assume text segments to be 4-byte aligned */
> + should_shalign = 4;
> else {
> /* search in list with special alignment */
> - k = 0;
> - while (special_list[k].align && strstarts(sec, special_list[k].name)) {
> - should_shalign = special_list[k].align;
> - break;
> + for (k = 0; special_list[k].align; k++) {
> + if (strstarts(sec, special_list[k].name)) {
> + should_shalign = special_list[k].align;
> + break;
> + }
> }
> }
>
> Helge Deller (4):
> linux/export: Fix alignment for 64-bit ksymtab entries
> modules: Ensure 64-bit alignment on __ksymtab_* sections
> vmlinux.lds.h: Fix alignment for __ksymtab*, __kcrctab_* and
> .pci_fixup sections
> modules: Add missing entry for __ex_table
>
> include/asm-generic/vmlinux.lds.h | 5 +++++
> include/linux/export-internal.h | 5 ++++-
> scripts/module.lds.S | 9 +++++----
> 3 files changed, 14 insertions(+), 5 deletions(-)
>
> --
> 2.41.0
>
--
Best Regards
Masahiro Yamada
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