[PATCH v3 mips-next 0/7] MIPS: vmlinux.lds.S sections fixes & cleanup

[PATCH v3 mips-next 0/7] MIPS: vmlinux.lds.S sections fixes & cleanup

[Alexander Lobakin]

This series hunts the problems discovered after manual enabling of
ARCH_WANT_LD_ORPHAN_WARN. Notably:
 - adds the missing PAGE_ALIGNED_DATA() section affecting VDSO
   placement (marked for stable);
 - properly stops .eh_frame section generation.

Compile and runtime tested on MIPS32R2 CPS board with no issues
using two different toolkits:
 - Binutils 2.35.1, GCC 10.2.0;
 - LLVM stack 11.0.0.

Since v2 [1]:
 - stop discarding .eh_frame and just prevent it from generating
   (Kees);
 - drop redundant sections assertions (Fangrui);
 - place GOT table in .text instead of asserting as it's not empty
   when building with LLVM (Nathan);
 - catch compound literals in generic definitions when building with
   LD_DEAD_CODE_DATA_ELIMINATION (Kees);
 - collect two Reviewed-bys (Kees).

Since v1 [0]:
 - catch .got entries too as LLD may produce it (Nathan);
 - check for unwanted sections to be zero-sized instead of
   discarding (Fangrui).

[0] https://lore.kernel.org/linux-mips/20210104121729.46981-1-alobakin@pm.me
[1] https://lore.kernel.org/linux-mips/20210106200713.31840-1-alobakin@pm.me

Alexander Lobakin (7):
  MIPS: vmlinux.lds.S: add missing PAGE_ALIGNED_DATA() section
  MIPS: vmlinux.lds.S: add ".gnu.attributes" to DISCARDS
  MIPS: properly stop .eh_frame generation
  MIPS: vmlinux.lds.S: catch bad .rel.dyn at link time
  MIPS: vmlinux.lds.S: explicitly declare .got table
  vmlinux.lds.h: catch compound literals into data and BSS
  MIPS: select ARCH_WANT_LD_ORPHAN_WARN

 arch/mips/Kconfig                 |  1 +
 arch/mips/include/asm/asm.h       | 18 ++++++++++++++++++
 arch/mips/kernel/vmlinux.lds.S    | 15 ++++++++++++++-
 include/asm-generic/vmlinux.lds.h |  6 +++---
 4 files changed, 36 insertions(+), 4 deletions(-)

-- 
2.30.0

[PATCH v3 mips-next 1/7] MIPS: vmlinux.lds.S: add missing PAGE_ALIGNED_DATA() section

[Alexander Lobakin]

MIPS uses its own declaration of rwdata, and thus it should be kept
in sync with the asm-generic one. Currently PAGE_ALIGNED_DATA() is
missing from the linker script, which emits the following ld
warnings:

mips-alpine-linux-musl-ld: warning: orphan section
`.data..page_aligned' from `arch/mips/kernel/vdso.o' being placed
in section `.data..page_aligned'
mips-alpine-linux-musl-ld: warning: orphan section
`.data..page_aligned' from `arch/mips/vdso/vdso-image.o' being placed
in section `.data..page_aligned'

Add the necessary declaration, so the mentioned structures will be
placed in vmlinux as intended:

ffffffff80630580 D __end_once
ffffffff80630580 D __start___dyndbg
ffffffff80630580 D __start_once
ffffffff80630580 D __stop___dyndbg
ffffffff80634000 d mips_vdso_data
ffffffff80638000 d vdso_data
ffffffff80638580 D _gp
ffffffff8063c000 T __init_begin
ffffffff8063c000 D _edata
ffffffff8063c000 T _sinittext

->

ffffffff805a4000 D __end_init_task
ffffffff805a4000 D __nosave_begin
ffffffff805a4000 D __nosave_end
ffffffff805a4000 d mips_vdso_data
ffffffff805a8000 d vdso_data
ffffffff805ac000 D mmlist_lock
ffffffff805ac080 D tasklist_lock

Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO")
Cc: stable@vger.kernel.org # 4.4+
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 arch/mips/kernel/vmlinux.lds.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 5e97e9d02f98..83e27a181206 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -90,6 +90,7 @@ SECTIONS
 
 		INIT_TASK_DATA(THREAD_SIZE)
 		NOSAVE_DATA
+		PAGE_ALIGNED_DATA(PAGE_SIZE)
 		CACHELINE_ALIGNED_DATA(1 << CONFIG_MIPS_L1_CACHE_SHIFT)
 		READ_MOSTLY_DATA(1 << CONFIG_MIPS_L1_CACHE_SHIFT)
 		DATA_DATA
-- 
2.30.0

[PATCH v3 mips-next 2/7] MIPS: vmlinux.lds.S: add ".gnu.attributes" to DISCARDS

[Alexander Lobakin]

Discard GNU attributes (MIPS FP type, GNU Hash etc.) at link time
as kernel doesn't use it at all.
Solves a dozen of the following ld warnings (one per every file):

mips-alpine-linux-musl-ld: warning: orphan section `.gnu.attributes'
from `arch/mips/kernel/head.o' being placed in section
`.gnu.attributes'
mips-alpine-linux-musl-ld: warning: orphan section `.gnu.attributes'
from `init/main.o' being placed in section `.gnu.attributes'

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 arch/mips/kernel/vmlinux.lds.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 83e27a181206..16468957cba2 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -221,6 +221,7 @@ SECTIONS
 		/* ABI crap starts here */
 		*(.MIPS.abiflags)
 		*(.MIPS.options)
+		*(.gnu.attributes)
 		*(.options)
 		*(.pdr)
 		*(.reginfo)
-- 
2.30.0

[PATCH v3 mips-next 3/7] MIPS: properly stop .eh_frame generation

[Alexander Lobakin]

Commit 866b6a89c6d1 ("MIPS: Add DWARF unwinding to assembly") added
-fno-asynchronous-unwind-tables to KBUILD_CFLAGS to prevent compiler
from emitting .eh_frame symbols.
However, as MIPS heavily uses CFI, that's not enough. Use the
approach taken for x86 (as it also uses CFI) and explicitly put CFI
symbols into the .debug_frame section (except for VDSO).
This allows us to drop .eh_frame from DISCARDS as it's no longer
being generated.

Fixes: 866b6a89c6d1 ("MIPS: Add DWARF unwinding to assembly")
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 arch/mips/include/asm/asm.h    | 18 ++++++++++++++++++
 arch/mips/kernel/vmlinux.lds.S |  1 -
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/asm.h b/arch/mips/include/asm/asm.h
index 3682d1a0bb80..908f6d6ae24b 100644
--- a/arch/mips/include/asm/asm.h
+++ b/arch/mips/include/asm/asm.h
@@ -20,10 +20,27 @@
 #include <asm/sgidefs.h>
 #include <asm/asm-eva.h>
 
+#ifndef __VDSO__
+/*
+ * Emit CFI data in .debug_frame sections, not .eh_frame sections.
+ * We don't do DWARF unwinding at runtime, so only the offline DWARF
+ * information is useful to anyone. Note we should change this if we
+ * ever decide to enable DWARF unwinding at runtime.
+ */
+#define CFI_SECTIONS	.cfi_sections .debug_frame
+#else
+ /*
+  * For the vDSO, emit both runtime unwind information and debug
+  * symbols for the .dbg file.
+  */
+#define CFI_SECTIONS	.cfi_sections .debug_frame, .eh_frame
+#endif
+
 /*
  * LEAF - declare leaf routine
  */
 #define LEAF(symbol)					\
+		CFI_SECTIONS;				\
 		.globl	symbol;				\
 		.align	2;				\
 		.type	symbol, @function;		\
@@ -36,6 +53,7 @@ symbol:		.frame	sp, 0, ra;			\
  * NESTED - declare nested routine entry point
  */
 #define NESTED(symbol, framesize, rpc)			\
+		CFI_SECTIONS;				\
 		.globl	symbol;				\
 		.align	2;				\
 		.type	symbol, @function;		\
diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 16468957cba2..0f4e46ea4458 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -225,6 +225,5 @@ SECTIONS
 		*(.options)
 		*(.pdr)
 		*(.reginfo)
-		*(.eh_frame)
 	}
 }
-- 
2.30.0

[PATCH v3 mips-next 4/7] MIPS: vmlinux.lds.S: catch bad .rel.dyn at link time

[Alexander Lobakin]

Catch any symbols placed in .rel.dyn and check for these sections
to be zero-sized at link time.
Eliminates following ld warning:

mips-alpine-linux-musl-ld: warning: orphan section `.rel.dyn'
from `init/main.o' being placed in section `.rel.dyn'

Adopted from x86/kernel/vmlinux.lds.S.

Suggested-by: Fangrui Song <maskray@google.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 arch/mips/kernel/vmlinux.lds.S | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 0f4e46ea4458..0f736d60d43e 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -226,4 +226,15 @@ SECTIONS
 		*(.pdr)
 		*(.reginfo)
 	}
+
+	/*
+	 * Sections that should stay zero sized, which is safer to
+	 * explicitly check instead of blindly discarding.
+	 */
+
+	.rel.dyn : {
+		*(.rel.*)
+		*(.rel_*)
+	}
+	ASSERT(SIZEOF(.rel.dyn) == 0, "Unexpected run-time relocations (.rel) detected!")
 }
-- 
2.30.0

[PATCH v3 mips-next 5/7] MIPS: vmlinux.lds.S: explicitly declare .got table

[Alexander Lobakin]

LLVM stack generates GOT table when building the kernel:

ld.lld: warning: <internal>:(.got) is being placed in '.got'

According to the debug assertions, it's not zero-sized and thus
can't be handled the same way as .rel.dyn (like it's done for x86).
Use the ARM/ARM64 path here and place it at the end of .text section.

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 arch/mips/kernel/vmlinux.lds.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index 0f736d60d43e..4709959f6985 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -69,6 +69,7 @@ SECTIONS
 		*(.text.*)
 		*(.fixup)
 		*(.gnu.warning)
+		*(.got)
 	} :text = 0
 	_etext = .;	/* End of text section */
 
-- 
2.30.0

[PATCH v3 mips-next 6/7] vmlinux.lds.h: catch compound literals into data and BSS

[Alexander Lobakin]

When building kernel with LD_DEAD_CODE_DATA_ELIMINATION, LLVM stack
generates separate sections for compound literals, just like in case
with enabled LTO [0]:

ld.lld: warning: drivers/built-in.a(mtd/nand/spi/gigadevice.o):
(.data..compoundliteral.14) is being placed in
'.data..compoundliteral.14'
ld.lld: warning: drivers/built-in.a(mtd/nand/spi/gigadevice.o):
(.data..compoundliteral.15) is being placed in
'.data..compoundliteral.15'
ld.lld: warning: drivers/built-in.a(mtd/nand/spi/gigadevice.o):
(.data..compoundliteral.16) is being placed in
'.data..compoundliteral.16'
ld.lld: warning: drivers/built-in.a(mtd/nand/spi/gigadevice.o):
(.data..compoundliteral.17) is being placed in
'.data..compoundliteral.17'

[...]

Handle this by adding the related sections to generic definitions
as suggested by Sami [0].

[0] https://lore.kernel.org/lkml/20201211184633.3213045-3-samitolvanen@google.com

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 include/asm-generic/vmlinux.lds.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index b2b3d81b1535..5f2f5b1db84f 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -95,10 +95,10 @@
  */
 #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
-#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
+#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral*
 #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
-#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
-#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
+#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L*
+#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral*
 #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
 #else
 #define TEXT_MAIN .text
-- 
2.30.0

[PATCH v3 mips-next 7/7] MIPS: select ARCH_WANT_LD_ORPHAN_WARN

[Alexander Lobakin]

Now, after that all the sections are explicitly described and
declared in vmlinux.lds.S, we can enable ld orphan warnings to
prevent from missing any new sections in future.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 arch/mips/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index d68df1febd25..d3e64cc0932b 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -18,6 +18,7 @@ config MIPS
 	select ARCH_USE_QUEUED_SPINLOCKS
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
 	select ARCH_WANT_IPC_PARSE_VERSION
+	select ARCH_WANT_LD_ORPHAN_WARN
 	select BUILDTIME_TABLE_SORT
 	select CLONE_BACKWARDS
 	select CPU_NO_EFFICIENT_FFS if (TARGET_ISA_REV < 1)
-- 
2.30.0

Re: [PATCH v3 mips-next 0/7] MIPS: vmlinux.lds.S sections fixes & cleanup

[Alexander Lobakin]

> This series hunts the problems discovered after manual enabling of
> ARCH_WANT_LD_ORPHAN_WARN. Notably:
>  - adds the missing PAGE_ALIGNED_DATA() section affecting VDSO
>    placement (marked for stable);
>  - properly stops .eh_frame section generation.
> 
> Compile and runtime tested on MIPS32R2 CPS board with no issues
> using two different toolkits:
>  - Binutils 2.35.1, GCC 10.2.0;
>  - LLVM stack 11.0.0.
> 
> Since v2 [1]:
>  - stop discarding .eh_frame and just prevent it from generating
>    (Kees);
>  - drop redundant sections assertions (Fangrui);
>  - place GOT table in .text instead of asserting as it's not empty
>    when building with LLVM (Nathan);
>  - catch compound literals in generic definitions when building with
>    LD_DEAD_CODE_DATA_ELIMINATION (Kees);
>  - collect two Reviewed-bys (Kees).
> 
> Since v1 [0]:
>  - catch .got entries too as LLD may produce it (Nathan);
>  - check for unwanted sections to be zero-sized instead of
>    discarding (Fangrui).
> 
> [0] https://lore.kernel.org/linux-mips/20210104121729.46981-1-alobakin@pm.me
> [1] https://lore.kernel.org/linux-mips/20210106200713.31840-1-alobakin@pm.me
> 
> Alexander Lobakin (7):
>   MIPS: vmlinux.lds.S: add missing PAGE_ALIGNED_DATA() section
>   MIPS: vmlinux.lds.S: add ".gnu.attributes" to DISCARDS
>   MIPS: properly stop .eh_frame generation

Well, GNU fails to build VDSO with this patch... Sorry, sending
v4 now.

>   MIPS: vmlinux.lds.S: catch bad .rel.dyn at link time
>   MIPS: vmlinux.lds.S: explicitly declare .got table
>   vmlinux.lds.h: catch compound literals into data and BSS
>   MIPS: select ARCH_WANT_LD_ORPHAN_WARN
> 
>  arch/mips/Kconfig                 |  1 +
>  arch/mips/include/asm/asm.h       | 18 ++++++++++++++++++
>  arch/mips/kernel/vmlinux.lds.S    | 15 ++++++++++++++-
>  include/asm-generic/vmlinux.lds.h |  6 +++---
>  4 files changed, 36 insertions(+), 4 deletions(-)
> 
> --
> 2.30.0

Al