[PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of .text too

[PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of .text too

[Nick Desaulniers]

ld.bfd's internal linker script considers .text.hot AND .text.hot.* to
be part of .text, as well as .text.unlikely and .text.unlikely.*. ld.lld
will produce .text.hot.*/.text.unlikely.* sections. Make sure to group
these together.  Otherwise these orphan sections may be placed outside
of the the _stext/_etext boundaries.

Cc: stable@vger.kernel.org
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
Link: https://reviews.llvm.org/D79600
Reported-by: Jian Cai <jiancai@google.com>
Debugged-by: Luis Lozano <llozano@google.com>
Suggested-by: Fāng-ruì Sòng <maskray@google.com>
Tested-by: Luis Lozano <llozano@google.com>
Tested-by: Manoj Gupta <manojgupta@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 include/asm-generic/vmlinux.lds.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index d7c7c7f36c4a..fe5aaef169e3 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -560,7 +560,9 @@
  */
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
-		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
+		*(.text.hot .text.hot.*)				\
+		*(TEXT_MAIN .text.fixup)				\
+		*(.text.unlikely .text.unlikely.*)			\
 		NOINSTR_TEXT						\
 		*(.text..refcount)					\
 		*(.ref.text)						\
-- 
2.27.0.290.gba653c62da-goog

Re: [PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of .text too

[Fāng-ruì Sòng]

On 2020-06-17, Nick Desaulniers wrote:
>ld.bfd's internal linker script considers .text.hot AND .text.hot.* to
>be part of .text, as well as .text.unlikely and .text.unlikely.*.

>ld.lld will produce .text.hot.*/.text.unlikely.* sections.

Correction to this sentence. lld is not relevant here.

-ffunction-sections combined with profile-guided optimization can
produce .text.hot.* .text.unlikely.* sections.  Newer clang may produce
.text.hot. .text.unlikely. (without suffix, but with a trailing dot)
when -fno-unique-section-names is specified, as an optimization to make
.strtab smaller.

We've already seen that GCC can place main in .text.startup without
-ffunction-sections. There may be other non -ffunction-sections cases
for .text.hot.* or .text.unlikely.*. So it is definitely a good idea to
be more specific even if we don't care about -ffunction-sections for
now.

>Make sure to group these together.  Otherwise these orphan sections may
>be placed outside of the the _stext/_etext boundaries.
>
>Cc: stable@vger.kernel.org
>Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
>Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
>Link: https://reviews.llvm.org/D79600
>Reported-by: Jian Cai <jiancai@google.com>
>Debugged-by: Luis Lozano <llozano@google.com>
>Suggested-by: Fāng-ruì Sòng <maskray@google.com>
>Tested-by: Luis Lozano <llozano@google.com>
>Tested-by: Manoj Gupta <manojgupta@google.com>
>Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>---
> include/asm-generic/vmlinux.lds.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>index d7c7c7f36c4a..fe5aaef169e3 100644
>--- a/include/asm-generic/vmlinux.lds.h
>+++ b/include/asm-generic/vmlinux.lds.h
>@@ -560,7 +560,9 @@
>  */
> #define TEXT_TEXT							\
> 		ALIGN_FUNCTION();					\
>-		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
>+		*(.text.hot .text.hot.*)				\
>+		*(TEXT_MAIN .text.fixup)				\
>+		*(.text.unlikely .text.unlikely.*)			\
> 		NOINSTR_TEXT						\
> 		*(.text..refcount)					\
> 		*(.ref.text)						\
>-- 
>2.27.0.290.gba653c62da-goog
>

Re: [PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of .text too

[Nick Desaulniers]

On Wed, Jun 17, 2020 at 2:27 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>
>
> On 2020-06-17, Nick Desaulniers wrote:
> >ld.bfd's internal linker script considers .text.hot AND .text.hot.* to
> >be part of .text, as well as .text.unlikely and .text.unlikely.*.
>
> >ld.lld will produce .text.hot.*/.text.unlikely.* sections.
>
> Correction to this sentence. lld is not relevant here.
>
> -ffunction-sections combined with profile-guided optimization can
> produce .text.hot.* .text.unlikely.* sections.  Newer clang may produce
> .text.hot. .text.unlikely. (without suffix, but with a trailing dot)
> when -fno-unique-section-names is specified, as an optimization to make
> .strtab smaller.

Then why was the bug report reporting https://reviews.llvm.org/D79600
as the result of a bisection, if LLD is not relevant?  Was the
bisection wrong?

The upstream report wasn't initially public, for no good reason.  So I
didn't include it, but if we end up taking v1, this should have

Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760

The kernel doesn't use -fno-unique-section-names; is that another flag
that's added by CrOS' compiler wrapper?
https://source.chromium.org/chromiumos/chromiumos/codesearch/+/master:src/third_party/toolchain-utils/compiler_wrapper/config.go;l=110
Looks like no.  It doesn't use `-fno-unique-section-names` or
`-ffunction-sections`.


>
> We've already seen that GCC can place main in .text.startup without
> -ffunction-sections. There may be other non -ffunction-sections cases
> for .text.hot.* or .text.unlikely.*. So it is definitely a good idea to
> be more specific even if we don't care about -ffunction-sections for
> now.
>
> >Make sure to group these together.  Otherwise these orphan sections may
> >be placed outside of the the _stext/_etext boundaries.
> >
> >Cc: stable@vger.kernel.org
> >Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
> >Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
> >Link: https://reviews.llvm.org/D79600
> >Reported-by: Jian Cai <jiancai@google.com>
> >Debugged-by: Luis Lozano <llozano@google.com>
> >Suggested-by: Fāng-ruì Sòng <maskray@google.com>
> >Tested-by: Luis Lozano <llozano@google.com>
> >Tested-by: Manoj Gupta <manojgupta@google.com>
> >Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> >---
> > include/asm-generic/vmlinux.lds.h | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> >diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> >index d7c7c7f36c4a..fe5aaef169e3 100644
> >--- a/include/asm-generic/vmlinux.lds.h
> >+++ b/include/asm-generic/vmlinux.lds.h
> >@@ -560,7 +560,9 @@
> >  */
> > #define TEXT_TEXT                                                     \
> >               ALIGN_FUNCTION();                                       \
> >-              *(.text.hot TEXT_MAIN .text.fixup .text.unlikely)       \
> >+              *(.text.hot .text.hot.*)                                \
> >+              *(TEXT_MAIN .text.fixup)                                \
> >+              *(.text.unlikely .text.unlikely.*)                      \
> >               NOINSTR_TEXT                                            \
> >               *(.text..refcount)                                      \
> >               *(.ref.text)                                            \
> >--
> >2.27.0.290.gba653c62da-goog
> >



--
Thanks,
~Nick Desaulniers

Re: [PATCH] vmlinux.lds: consider .text.{hot|unlikely}.* part of .text too

[Fāng-ruì Sòng]

On 2020-06-22, Nick Desaulniers wrote:
>On Wed, Jun 17, 2020 at 2:27 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>>
>>
>> On 2020-06-17, Nick Desaulniers wrote:
>> >ld.bfd's internal linker script considers .text.hot AND .text.hot.* to
>> >be part of .text, as well as .text.unlikely and .text.unlikely.*.
>>
>> >ld.lld will produce .text.hot.*/.text.unlikely.* sections.
>>
>> Correction to this sentence. lld is not relevant here.
>>
>> -ffunction-sections combined with profile-guided optimization can
>> produce .text.hot.* .text.unlikely.* sections.  Newer clang may produce
>> .text.hot. .text.unlikely. (without suffix, but with a trailing dot)
>> when -fno-unique-section-names is specified, as an optimization to make
>> .strtab smaller.
>
>Then why was the bug report reporting https://reviews.llvm.org/D79600
>as the result of a bisection, if LLD is not relevant?  Was the
>bisection wrong?

https://reviews.llvm.org/D79600 is an LLVM codegen change, unrelated to LLD..
(As described in the patch, LLD's -z keep-text-section-prefix only
recognizes ".text.exit.*", not ".text.exit")

>The upstream report wasn't initially public, for no good reason.  So I
>didn't include it, but if we end up taking v1, this should have
>
>Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760

Thanks for making it public.

>The kernel doesn't use -fno-unique-section-names; is that another flag
>that's added by CrOS' compiler wrapper?
>https://source.chromium.org/chromiumos/chromiumos/codesearch/+/master:src/third_party/toolchain-utils/compiler_wrapper/config.go;l=110
>Looks like no.  It doesn't use `-fno-unique-section-names` or
>`-ffunction-sections`.

-fno-unique-section-names is a very rare option. It is not supported by GCC (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95095 ).
clang users use it very rarely, probably because not many people care
about additional strings taken by section names ".text.hot.a" ".text.hot.b" ".text.hot.c"
in the string table ".strtab" (clang since some point of 2018 uses
.strtab instead of .shstrtab which enables more string sharing).

>
>

>
>>
>> We've already seen that GCC can place main in .text.startup without
>> -ffunction-sections. There may be other non -ffunction-sections cases
>> for .text.hot.* or .text.unlikely.*. So it is definitely a good idea to
>> be more specific even if we don't care about -ffunction-sections for
>> now.
>>
>> >Make sure to group these together.  Otherwise these orphan sections may
>> >be placed outside of the the _stext/_etext boundaries.
>> >
>> >Cc: stable@vger.kernel.org
>> >Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
>> >Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
>> >Link: https://reviews.llvm.org/D79600
>> >Reported-by: Jian Cai <jiancai@google.com>
>> >Debugged-by: Luis Lozano <llozano@google.com>
>> >Suggested-by: Fāng-ruì Sòng <maskray@google.com>
>> >Tested-by: Luis Lozano <llozano@google.com>
>> >Tested-by: Manoj Gupta <manojgupta@google.com>
>> >Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>> >---
>> > include/asm-generic/vmlinux.lds.h | 4 +++-
>> > 1 file changed, 3 insertions(+), 1 deletion(-)
>> >
>> >diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>> >index d7c7c7f36c4a..fe5aaef169e3 100644
>> >--- a/include/asm-generic/vmlinux.lds.h
>> >+++ b/include/asm-generic/vmlinux.lds.h
>> >@@ -560,7 +560,9 @@
>> >  */
>> > #define TEXT_TEXT                                                     \
>> >               ALIGN_FUNCTION();                                       \
>> >-              *(.text.hot TEXT_MAIN .text.fixup .text.unlikely)       \
>> >+              *(.text.hot .text.hot.*)                                \
>> >+              *(TEXT_MAIN .text.fixup)                                \
>> >+              *(.text.unlikely .text.unlikely.*)                      \
>> >               NOINSTR_TEXT                                            \
>> >               *(.text..refcount)                                      \
>> >               *(.ref.text)                                            \
>> >--
>> >2.27.0.290.gba653c62da-goog
>> >
>
>
>
>--
>Thanks,
>~Nick Desaulniers

[PATCH v2] vmlinux.lds: add PGO and AutoFDO input sections

[Nick Desaulniers]

Basically, consider .text.{hot|unlikely|unknown}.* part of .text, too.

When compiling with profiling information (collected via PGO
instrumentations or AutoFDO sampling), Clang will separate code into
.text.hot, .text.unlikely, or .text.unknown sections based on profiling
information. After D79600 (clang-11), these sections will have a
trailing `.` suffix, ie.  .text.hot., .text.unlikely., .text.unknown..

When using -ffunction-sections together with profiling infomation,
either explicitly (FGKASLR) or implicitly (LTO), code may be placed in
sections following the convention:
.text.hot.<foo>, .text.unlikely.<bar>, .text.unknown.<baz>
where <foo>, <bar>, and <baz> are functions.  (This produces one section
per function; we generally try to merge these all back via linker script
so that we don't have 50k sections).

For the above cases, we need to teach our linker scripts that such
sections might exist and that we'd explicitly like them grouped
together, otherwise we can wind up with code outside of the
_stext/_etext boundaries that might not be mapped properly for some
architectures, resulting in boot failures.

If the linker script is not told about possible input sections, then
where the section is placed as output is a heuristic-laiden mess that's
non-portable between linkers (ie. BFD and LLD), and has resulted in many
hard to debug bugs.  Kees Cook is working on cleaning this up by adding
--orphan-handling=warn linker flag used in ARCH=powerpc to additional
architectures. In the case of linker scripts, borrowing from the Zen of
Python: explicit is better than implicit.

Also, ld.bfd's internal linker script considers .text.hot AND
.text.hot.* to be part of .text, as well as .text.unlikely and
.text.unlikely.*. I didn't see support for .text.unknown.*, and didn't
see Clang producing such code in our kernel builds, but I see code in
LLVM that can produce such section names if profiling information is
missing. That may point to a larger issue with generating or collecting
profiles, but I would much rather be safe and explicit than have to
debug yet another issue related to orphan section placement.

Cc: stable@vger.kernel.org
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
Link: https://reviews.llvm.org/D79600
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760
Reported-by: Jian Cai <jiancai@google.com>
Debugged-by: Luis Lozano <llozano@google.com>
Suggested-by: Fāng-ruì Sòng <maskray@google.com>
Tested-by: Luis Lozano <llozano@google.com>
Tested-by: Manoj Gupta <manojgupta@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V1 -> V2:
* Add .text.unknown.*.  It's not strictly necessary for us yet, but I
  really worry that it could become a problem for us. Either way, I'm
  happy to drop for a V3, but I'm suggesting we not.
* Beef up commit message.
* Drop references to LLD; the LLVM change had nothing to do with LLD.
  I've realized I have a Pavlovian-response to changes from Fāng-ruì
  that I associate with LLD.  I'm seeking professional help for my
  ailment. Forgive me.
* Add link to now public CrOS bug.

 include/asm-generic/vmlinux.lds.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index d7c7c7f36c4a..245c1af4c057 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -560,7 +560,10 @@
  */
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
-		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
+		*(.text.hot .text.hot.*)				\
+		*(TEXT_MAIN .text.fixup)				\
+		*(.text.unlikely .text.unlikely.*)			\
+		*(.text.unknown .text.unknown.*)			\
 		NOINSTR_TEXT						\
 		*(.text..refcount)					\
 		*(.ref.text)						\
-- 
2.27.0.111.gc72c7da667-goog

Re: [PATCH v2] vmlinux.lds: add PGO and AutoFDO input sections

[Nick Desaulniers]

Hi Arnd,
I usually wait longer to bump threads for review, but we have a
holiday in the US so we're off tomorrow and Friday.
scripts/get_maintainer.pl recommend you for this patch.  Would you
take a look at it for us, please?

On Thu, Jun 25, 2020 at 11:48 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Basically, consider .text.{hot|unlikely|unknown}.* part of .text, too.
>
> When compiling with profiling information (collected via PGO
> instrumentations or AutoFDO sampling), Clang will separate code into
> .text.hot, .text.unlikely, or .text.unknown sections based on profiling
> information. After D79600 (clang-11), these sections will have a
> trailing `.` suffix, ie.  .text.hot., .text.unlikely., .text.unknown..
>
> When using -ffunction-sections together with profiling infomation,
> either explicitly (FGKASLR) or implicitly (LTO), code may be placed in
> sections following the convention:
> .text.hot.<foo>, .text.unlikely.<bar>, .text.unknown.<baz>
> where <foo>, <bar>, and <baz> are functions.  (This produces one section
> per function; we generally try to merge these all back via linker script
> so that we don't have 50k sections).
>
> For the above cases, we need to teach our linker scripts that such
> sections might exist and that we'd explicitly like them grouped
> together, otherwise we can wind up with code outside of the
> _stext/_etext boundaries that might not be mapped properly for some
> architectures, resulting in boot failures.
>
> If the linker script is not told about possible input sections, then
> where the section is placed as output is a heuristic-laiden mess that's
> non-portable between linkers (ie. BFD and LLD), and has resulted in many
> hard to debug bugs.  Kees Cook is working on cleaning this up by adding
> --orphan-handling=warn linker flag used in ARCH=powerpc to additional
> architectures. In the case of linker scripts, borrowing from the Zen of
> Python: explicit is better than implicit.
>
> Also, ld.bfd's internal linker script considers .text.hot AND
> .text.hot.* to be part of .text, as well as .text.unlikely and
> .text.unlikely.*. I didn't see support for .text.unknown.*, and didn't
> see Clang producing such code in our kernel builds, but I see code in
> LLVM that can produce such section names if profiling information is
> missing. That may point to a larger issue with generating or collecting
> profiles, but I would much rather be safe and explicit than have to
> debug yet another issue related to orphan section placement.
>
> Cc: stable@vger.kernel.org
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
> Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
> Link: https://reviews.llvm.org/D79600
> Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760
> Reported-by: Jian Cai <jiancai@google.com>
> Debugged-by: Luis Lozano <llozano@google.com>
> Suggested-by: Fāng-ruì Sòng <maskray@google.com>
> Tested-by: Luis Lozano <llozano@google.com>
> Tested-by: Manoj Gupta <manojgupta@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes V1 -> V2:
> * Add .text.unknown.*.  It's not strictly necessary for us yet, but I
>   really worry that it could become a problem for us. Either way, I'm
>   happy to drop for a V3, but I'm suggesting we not.
> * Beef up commit message.
> * Drop references to LLD; the LLVM change had nothing to do with LLD.
>   I've realized I have a Pavlovian-response to changes from Fāng-ruì
>   that I associate with LLD.  I'm seeking professional help for my
>   ailment. Forgive me.
> * Add link to now public CrOS bug.
>
>  include/asm-generic/vmlinux.lds.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index d7c7c7f36c4a..245c1af4c057 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -560,7 +560,10 @@
>   */
>  #define TEXT_TEXT                                                      \
>                 ALIGN_FUNCTION();                                       \
> -               *(.text.hot TEXT_MAIN .text.fixup .text.unlikely)       \
> +               *(.text.hot .text.hot.*)                                \
> +               *(TEXT_MAIN .text.fixup)                                \
> +               *(.text.unlikely .text.unlikely.*)                      \
> +               *(.text.unknown .text.unknown.*)                        \
>                 NOINSTR_TEXT                                            \
>                 *(.text..refcount)                                      \
>                 *(.ref.text)                                            \
> --
> 2.27.0.111.gc72c7da667-goog
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2] vmlinux.lds: add PGO and AutoFDO input sections

[Arnd Bergmann]

On Wed, Jul 1, 2020 at 11:54 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> Hi Arnd,
> I usually wait longer to bump threads for review, but we have a
> holiday in the US so we're off tomorrow and Friday.
> scripts/get_maintainer.pl recommend you for this patch.  Would you
> take a look at it for us, please?

Hi Nick

While I'm listed as the maintainer for include/asm-generic, linker scripts
are really not my expertise and I have no way of knowing whether the
change is good or not.

Your description looks very reasonable of course and I have no problem
with having someone else pick it up. You mentioned that Kees is already
looking at some related work and he's already done more changes to
this file than anyone else. If he can provide an Ack for this patch,
you can add mine as well to send it to akpm, or I can pick it up in the
asm-generic tree.

       Arnd

> On Thu, Jun 25, 2020 at 11:48 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > Basically, consider .text.{hot|unlikely|unknown}.* part of .text, too.
> >
> > When compiling with profiling information (collected via PGO
> > instrumentations or AutoFDO sampling), Clang will separate code into
> > .text.hot, .text.unlikely, or .text.unknown sections based on profiling
> > information. After D79600 (clang-11), these sections will have a
> > trailing `.` suffix, ie.  .text.hot., .text.unlikely., .text.unknown..
> >
> > When using -ffunction-sections together with profiling infomation,
> > either explicitly (FGKASLR) or implicitly (LTO), code may be placed in
> > sections following the convention:
> > .text.hot.<foo>, .text.unlikely.<bar>, .text.unknown.<baz>
> > where <foo>, <bar>, and <baz> are functions.  (This produces one section
> > per function; we generally try to merge these all back via linker script
> > so that we don't have 50k sections).
> >
> > For the above cases, we need to teach our linker scripts that such
> > sections might exist and that we'd explicitly like them grouped
> > together, otherwise we can wind up with code outside of the
> > _stext/_etext boundaries that might not be mapped properly for some
> > architectures, resulting in boot failures.
> >
> > If the linker script is not told about possible input sections, then
> > where the section is placed as output is a heuristic-laiden mess that's
> > non-portable between linkers (ie. BFD and LLD), and has resulted in many
> > hard to debug bugs.  Kees Cook is working on cleaning this up by adding
> > --orphan-handling=warn linker flag used in ARCH=powerpc to additional
> > architectures. In the case of linker scripts, borrowing from the Zen of
> > Python: explicit is better than implicit.
> >
> > Also, ld.bfd's internal linker script considers .text.hot AND
> > .text.hot.* to be part of .text, as well as .text.unlikely and
> > .text.unlikely.*. I didn't see support for .text.unknown.*, and didn't
> > see Clang producing such code in our kernel builds, but I see code in
> > LLVM that can produce such section names if profiling information is
> > missing. That may point to a larger issue with generating or collecting
> > profiles, but I would much rather be safe and explicit than have to
> > debug yet another issue related to orphan section placement.
> >
> > Cc: stable@vger.kernel.org
> > Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
> > Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
> > Link: https://reviews.llvm.org/D79600
> > Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760
> > Reported-by: Jian Cai <jiancai@google.com>
> > Debugged-by: Luis Lozano <llozano@google.com>
> > Suggested-by: Fāng-ruì Sòng <maskray@google.com>
> > Tested-by: Luis Lozano <llozano@google.com>
> > Tested-by: Manoj Gupta <manojgupta@google.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> > Changes V1 -> V2:
> > * Add .text.unknown.*.  It's not strictly necessary for us yet, but I
> >   really worry that it could become a problem for us. Either way, I'm
> >   happy to drop for a V3, but I'm suggesting we not.
> > * Beef up commit message.
> > * Drop references to LLD; the LLVM change had nothing to do with LLD.
> >   I've realized I have a Pavlovian-response to changes from Fāng-ruì
> >   that I associate with LLD.  I'm seeking professional help for my
> >   ailment. Forgive me.
> > * Add link to now public CrOS bug.
> >
> >  include/asm-generic/vmlinux.lds.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index d7c7c7f36c4a..245c1af4c057 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -560,7 +560,10 @@
> >   */
> >  #define TEXT_TEXT                                                      \
> >                 ALIGN_FUNCTION();                                       \
> > -               *(.text.hot TEXT_MAIN .text.fixup .text.unlikely)       \
> > +               *(.text.hot .text.hot.*)                                \
> > +               *(TEXT_MAIN .text.fixup)                                \
> > +               *(.text.unlikely .text.unlikely.*)                      \
> > +               *(.text.unknown .text.unknown.*)                        \
> >                 NOINSTR_TEXT                                            \
> >                 *(.text..refcount)                                      \
> >                 *(.ref.text)                                            \
> > --
> > 2.27.0.111.gc72c7da667-goog
> >

Re: [PATCH v2] vmlinux.lds: add PGO and AutoFDO input sections

[Kees Cook]

On Thu, Jul 02, 2020 at 10:19:40AM +0200, Arnd Bergmann wrote:
> On Wed, Jul 1, 2020 at 11:54 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> wrote:
> >
> > Hi Arnd,
> > I usually wait longer to bump threads for review, but we have a
> > holiday in the US so we're off tomorrow and Friday.
> > scripts/get_maintainer.pl recommend you for this patch.  Would you
> > take a look at it for us, please?
> 
> Hi Nick
> 
> While I'm listed as the maintainer for include/asm-generic, linker scripts
> are really not my expertise and I have no way of knowing whether the
> change is good or not.
> 
> Your description looks very reasonable of course and I have no problem
> with having someone else pick it up. You mentioned that Kees is already
> looking at some related work and he's already done more changes to
> this file than anyone else. If he can provide an Ack for this patch,
> you can add mine as well to send it to akpm, or I can pick it up in the
> asm-generic tree.

This looks good to me. Do you want me to carry it as part of the orphan
series? (It doesn't look like it'll collide, so that's not needed, but I
can if that makes things easier.)

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> 
>        Arnd
> 
> > On Thu, Jun 25, 2020 at 11:48 AM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > Basically, consider .text.{hot|unlikely|unknown}.* part of .text, too.
> > >
> > > When compiling with profiling information (collected via PGO
> > > instrumentations or AutoFDO sampling), Clang will separate code into
> > > .text.hot, .text.unlikely, or .text.unknown sections based on profiling
> > > information. After D79600 (clang-11), these sections will have a
> > > trailing `.` suffix, ie.  .text.hot., .text.unlikely., .text.unknown..
> > >
> > > When using -ffunction-sections together with profiling infomation,
> > > either explicitly (FGKASLR) or implicitly (LTO), code may be placed in
> > > sections following the convention:
> > > .text.hot.<foo>, .text.unlikely.<bar>, .text.unknown.<baz>
> > > where <foo>, <bar>, and <baz> are functions.  (This produces one section
> > > per function; we generally try to merge these all back via linker script
> > > so that we don't have 50k sections).
> > >
> > > For the above cases, we need to teach our linker scripts that such
> > > sections might exist and that we'd explicitly like them grouped
> > > together, otherwise we can wind up with code outside of the
> > > _stext/_etext boundaries that might not be mapped properly for some
> > > architectures, resulting in boot failures.
> > >
> > > If the linker script is not told about possible input sections, then
> > > where the section is placed as output is a heuristic-laiden mess that's
> > > non-portable between linkers (ie. BFD and LLD), and has resulted in many
> > > hard to debug bugs.  Kees Cook is working on cleaning this up by adding
> > > --orphan-handling=warn linker flag used in ARCH=powerpc to additional
> > > architectures. In the case of linker scripts, borrowing from the Zen of
> > > Python: explicit is better than implicit.
> > >
> > > Also, ld.bfd's internal linker script considers .text.hot AND
> > > .text.hot.* to be part of .text, as well as .text.unlikely and
> > > .text.unlikely.*. I didn't see support for .text.unknown.*, and didn't
> > > see Clang producing such code in our kernel builds, but I see code in
> > > LLVM that can produce such section names if profiling information is
> > > missing. That may point to a larger issue with generating or collecting
> > > profiles, but I would much rather be safe and explicit than have to
> > > debug yet another issue related to orphan section placement.
> > >
> > > Cc: stable@vger.kernel.org
> > > Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=add44f8d5c5c05e08b11e033127a744d61c26aee
> > > Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=1de778ed23ce7492c523d5850c6c6dbb34152655
> > > Link: https://reviews.llvm.org/D79600
> > > Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084760
> > > Reported-by: Jian Cai <jiancai@google.com>
> > > Debugged-by: Luis Lozano <llozano@google.com>
> > > Suggested-by: Fāng-ruì Sòng <maskray@google.com>
> > > Tested-by: Luis Lozano <llozano@google.com>
> > > Tested-by: Manoj Gupta <manojgupta@google.com>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > > ---
> > > Changes V1 -> V2:
> > > * Add .text.unknown.*.  It's not strictly necessary for us yet, but I
> > >   really worry that it could become a problem for us. Either way, I'm
> > >   happy to drop for a V3, but I'm suggesting we not.
> > > * Beef up commit message.
> > > * Drop references to LLD; the LLVM change had nothing to do with LLD.
> > >   I've realized I have a Pavlovian-response to changes from Fāng-ruì
> > >   that I associate with LLD.  I'm seeking professional help for my
> > >   ailment. Forgive me.
> > > * Add link to now public CrOS bug.
> > >
> > >  include/asm-generic/vmlinux.lds.h | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > > index d7c7c7f36c4a..245c1af4c057 100644
> > > --- a/include/asm-generic/vmlinux.lds.h
> > > +++ b/include/asm-generic/vmlinux.lds.h
> > > @@ -560,7 +560,10 @@
> > >   */
> > >  #define TEXT_TEXT                                                      \
> > >                 ALIGN_FUNCTION();                                       \
> > > -               *(.text.hot TEXT_MAIN .text.fixup .text.unlikely)       \
> > > +               *(.text.hot .text.hot.*)                                \
> > > +               *(TEXT_MAIN .text.fixup)                                \
> > > +               *(.text.unlikely .text.unlikely.*)                      \
> > > +               *(.text.unknown .text.unknown.*)                        \
> > >                 NOINSTR_TEXT                                            \
> > >                 *(.text..refcount)                                      \
> > >                 *(.ref.text)                                            \
> > > --
> > > 2.27.0.111.gc72c7da667-goog
> > >

-- 
Kees Cook

Re: [PATCH v2] vmlinux.lds: add PGO and AutoFDO input sections

[Nick Desaulniers]

On Thu, Jul 2, 2020 at 8:57 AM Kees Cook <keescook@chromium.org> wrote:
>
> This looks good to me. Do you want me to carry it as part of the orphan
> series? (It doesn't look like it'll collide, so that's not needed, but I
> can if that makes things easier.)
>
> Acked-by: Kees Cook <keescook@chromium.org>

If you would be so kind, I'd owe you yet another beer!
-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2] vmlinux.lds: add PGO and AutoFDO input sections

[Kees Cook]

On Wed, Jul 08, 2020 at 04:13:54PM -0700, Nick Desaulniers wrote:
> On Thu, Jul 2, 2020 at 8:57 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > This looks good to me. Do you want me to carry it as part of the orphan
> > series? (It doesn't look like it'll collide, so that's not needed, but I
> > can if that makes things easier.)
> >
> > Acked-by: Kees Cook <keescook@chromium.org>
> 
> If you would be so kind, I'd owe you yet another beer!

Yup! It's on my list; I've been clearing other stuff so I can do another
revision. (I want to move some things out of discard and into 0-size
asserts, and possibly collect Arvind's runtime relocations series too
(since it seems basically done but no x86 maintainers have snagged it).

-- 
Kees Cook