Messages in this thread Patch in this message | | | Date | Fri, 28 Apr 2023 21:06:11 +0000 | From | Fangrui Song <> | Subject | Re: [PATCH] arm64: lds: move .got section out of .text |
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On 2023-04-28, Ard Biesheuvel wrote: >On Fri, 28 Apr 2023 at 19:58, Fangrui Song <maskray@google.com> wrote: >> >> On 2023-04-28, Ard Biesheuvel wrote: >> >Hello Fangrui, >> >> Hello Ard, thank you for the rapid response. >> >> >On Fri, 28 Apr 2023 at 06:05, Fangrui Song <maskray@google.com> wrote: >> >> >> >> Currently, the .got section is placed within the output section .text. >> >> However, when .got is non-empty, the SHF_WRITE flag is set when linked >> >> by lld. GNU ld recognizes .text as a special section and ignores the >> >> SHF_WRITE flag. By renaming .text, we can also get the SHF_WRITE flag. >> >> >> >> Conventionally, the .got section is placed just before .got.plt (which >> >> should be empty and omitted in the kernel). Therefore, we move the .got >> >> section to a conventional location (between .text and .data) and remove >> >> the unneeded `. = ALIGN(16)`. >> >> >> >> Signed-off-by: Fangrui Song <maskray@google.com> >> >> --- >> >> arch/arm64/kernel/vmlinux.lds.S | 20 ++++++++++---------- >> >> 1 file changed, 10 insertions(+), 10 deletions(-) >> >> >> >> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S >> >> index b9202c2ee18e..2bcb3b30db41 100644 >> >> --- a/arch/arm64/kernel/vmlinux.lds.S >> >> +++ b/arch/arm64/kernel/vmlinux.lds.S >> >> @@ -181,18 +181,8 @@ SECTIONS >> >> KPROBES_TEXT >> >> HYPERVISOR_TEXT >> >> *(.gnu.warning) >> >> - . = ALIGN(16); >> >> - *(.got) /* Global offset table */ >> >> } >> >> >> >> - /* >> >> - * Make sure that the .got.plt is either completely empty or it >> >> - * contains only the lazy dispatch entries. >> >> - */ >> >> - .got.plt : { *(.got.plt) } >> >> - ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, >> >> - "Unexpected GOT/PLT entries detected!") >> >> - >> >> . = ALIGN(SEGMENT_ALIGN); >> >> _etext = .; /* End of text section */ >> >> >> >> @@ -247,6 +237,16 @@ SECTIONS >> >> >> >> . = ALIGN(SEGMENT_ALIGN); >> >> __inittext_end = .; >> >> + >> >> + .got : { *(.got) } >> > >> >This is the .init region, which gets freed and unmapped after boot. If >> >the GOT is non-empty, it needs to remain mapped, so we cannot place it >> >here. >> >> Thanks. I did not know the constraint. >> >> >We have the same issue with the .rodata section, which incorporates >> >variables marked as __ro_after_init, which are not const qualified. So >> >given that .rodata is already emitted as WA, and we cannot do anything >> >about that, let's move the GOT in there. >> >> Yes, writable .data..ro_after_init and __jump_table sections in >> include/asm-generic/vmlinux.lds.h (#define RO_DATA(align)) makes the >> output section .rodata writable. Perhaps this is very difficult to fix, >> and we will have writable .rodata for a long time. >> >> What do you think of moving .got/.got.plt immediately before .data? >> I want to place .got/.got.plt before the guaranteed-writable sections, >> not some sections which are "unfortunately" writable (.rodata, __modver, >> .hyp.rodata, .rodata.text, etc). >> >> For userspace programs, either linked with GNU ld or lld, .got/.got.plt >> are usually immediately before .data . >> > >I don't think that would be the right choice. > >We have five pseudo-segments in the kernel > >text (RX) >rodata (R) >inittext (RX) >initdata (RW) >data (RW) > >where the init ones disappear entirely when the boot completes. > >The GOT should not be modifiable, so it should not be in .data. So the >only appropriate 'segment' for the GOT is rodata > >Note that we don't use PIC codegen for the kernel, so all const >qualified data structures containing statically initialized global >pointer variables are emitted into .rodata as well, and relocated at >boot. So having the GOT in rodata too makes sense imho.
arm64's vmlinux is linked with -shared -Bsymbolic and .got has many entries that need to be relocated. .got is initially writable, and becomes read-only after relocation resolving. I think this property is significant enough to make it outside of any rodata segment.
(In the userspace .got is typically placed in PT_GNU_RELRO by using -z relro.)
If we remove inittext and initdata pseudo-segments which disappear after the boot, we have
text (RX) rodata (R) /// where is .got (RELRO) data (RW)
If we consider .got distinct from rodata, I think placing .got immediately after rodata or immediately before data is both fine?
> > > >> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S >> index b9202c2ee18e..48bd7c25b6ab 100644 >> --- a/arch/arm64/kernel/vmlinux.lds.S >> +++ b/arch/arm64/kernel/vmlinux.lds.S >> @@ -181,18 +181,8 @@ SECTIONS >> KPROBES_TEXT >> HYPERVISOR_TEXT >> *(.gnu.warning) >> - . = ALIGN(16); >> - *(.got) /* Global offset table */ >> } >> >> - /* >> - * Make sure that the .got.plt is either completely empty or it >> - * contains only the lazy dispatch entries. >> - */ >> - .got.plt : { *(.got.plt) } >> - ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, >> - "Unexpected GOT/PLT entries detected!") >> - >> . = ALIGN(SEGMENT_ALIGN); >> _etext = .; /* End of text section */ >> >> @@ -286,6 +276,15 @@ SECTIONS >> __initdata_end = .; >> __init_end = .; >> >> + .got : { *(.got) } >> + /* >> + * Make sure that the .got.plt is either completely empty or it >> + * contains only the lazy dispatch entries. >> + */ >> + .got.plt : { *(.got.plt) } >> + ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, >> + "Unexpected GOT/PLT entries detected!") >> + >> _data = .; >> _sdata = .; >> RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN) >> -- >> 2.40.1.495.gc816e09b53d-goog >> >> >> >> + /* >> >> + * Make sure that the .got.plt is either completely empty or it >> >> + * contains only the lazy dispatch entries. >> >> + */ >> >> + .got.plt : { *(.got.plt) } >> >> + ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, >> >> + "Unexpected GOT/PLT entries detected!") >> >> + >> >> __initdata_begin = .; >> >> >> >> init_idmap_pg_dir = .; >> >> -- >> >> 2.40.1.495.gc816e09b53d-goog >> >>
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