Re: [PATCH 0/4] Section alignment issues?
From: Masahiro Yamada
Date: Thu Dec 21 2023 - 10:45:27 EST
On Thu, Dec 21, 2023 at 10:40 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> On Thu, Nov 23, 2023 at 7:18 AM <deller@xxxxxxxxxx> wrote:
> >
> > From: Helge Deller <deller@xxxxxx>
> >
> > 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.
Sorry, I replied too early.
(Actually I replied without reading your modpost code).
Now, I understand what your checker is doing.
I did not test how many false positives are produced,
but it catches several suspicious mis-alignments.
However, I am not convinced with this warning.
+ 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);
+ }
Adding ALGIN() hides the real problem.
I think the real problem is that not enough alignment was requested
in the code.
For example, the right fix for ".initcall7.init" should be this:
diff --git a/include/linux/init.h b/include/linux/init.h
index 3fa3f6241350..650311e4b215 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -264,6 +264,7 @@ extern struct module __this_module;
#define ____define_initcall(fn, __stub, __name, __sec) \
__define_initcall_stub(__stub, fn) \
asm(".section \"" __sec "\", \"a\" \n" \
+ ".balign 4 \n" \
__stringify(__name) ": \n" \
".long " __stringify(__stub) " - . \n" \
".previous \n"); \
Then, "this section requires at least 4 byte alignment"
is recorded in the sh_addralign field.
Then, the rest is the linker's job.
We should not tweak the linker script.
--
Best Regards
Masahiro Yamada