[tip:,x86/entry] x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
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Message ID 161121327995.414.14890124942899525500.tip-bot2@tip-bot2
State New, archived
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  • [tip:,x86/entry] x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
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Commit Message

tip-bot2 for Peter Zijlstra Jan. 21, 2021, 7:14 a.m. UTC
The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
Gitweb:        https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
Author:        Andrea Righi <andrea.righi@canonical.com>
AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00

x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y

With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an
empty object file.

With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU
assembler doesn't generate a symbol table for empty object files and
objtool fails with the following error when a valid symbol table cannot
be found:

  arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table

To prevent this from happening, build thunk_$(BITS).o only if
CONFIG_PREEMPTION is enabled.

  BugLink: https://bugs.launchpad.net/bugs/1911359

Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390
---
 arch/x86/entry/Makefile   | 3 ++-
 arch/x86/entry/thunk_32.S | 2 --
 arch/x86/entry/thunk_64.S | 4 ----
 3 files changed, 2 insertions(+), 7 deletions(-)

Comments

Ingo Molnar Jan. 21, 2021, 7:49 a.m. UTC | #1
* tip-bot2 for Andrea Righi <tip-bot2@linutronix.de> wrote:

> The following commit has been merged into the x86/entry branch of tip:
> 
> Commit-ID:     e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> Gitweb:        https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> Author:        Andrea Righi <andrea.righi@canonical.com>
> AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00
> 
> x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
> 
> With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an
> empty object file.
> 
> With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU
> assembler doesn't generate a symbol table for empty object files and
> objtool fails with the following error when a valid symbol table cannot
> be found:
> 
>   arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table
> 
> To prevent this from happening, build thunk_$(BITS).o only if
> CONFIG_PREEMPTION is enabled.
> 
>   BugLink: https://bugs.launchpad.net/bugs/1911359
> 
> Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft")
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390

Hm, this fails to build on UML defconfig:

 /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_thunk':
 /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:34: undefined reference to `preempt_schedule'
 /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_notrace_thunk':
 /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:35: undefined reference to `preempt_schedule_notrace'

Thanks,

	Ingo
Andrea Righi Jan. 21, 2021, 8:51 a.m. UTC | #2
On Thu, Jan 21, 2021 at 08:49:28AM +0100, Ingo Molnar wrote:
> 
> * tip-bot2 for Andrea Righi <tip-bot2@linutronix.de> wrote:
> 
> > The following commit has been merged into the x86/entry branch of tip:
> > 
> > Commit-ID:     e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> > Gitweb:        https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> > Author:        Andrea Righi <andrea.righi@canonical.com>
> > AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
> > Committer:     Ingo Molnar <mingo@kernel.org>
> > CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00
> > 
> > x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
> > 
> > With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an
> > empty object file.
> > 
> > With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU
> > assembler doesn't generate a symbol table for empty object files and
> > objtool fails with the following error when a valid symbol table cannot
> > be found:
> > 
> >   arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table
> > 
> > To prevent this from happening, build thunk_$(BITS).o only if
> > CONFIG_PREEMPTION is enabled.
> > 
> >   BugLink: https://bugs.launchpad.net/bugs/1911359
> > 
> > Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft")
> > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390
> 
> Hm, this fails to build on UML defconfig:
> 
>  /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_thunk':
>  /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:34: undefined reference to `preempt_schedule'
>  /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_notrace_thunk':
>  /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:35: undefined reference to `preempt_schedule_notrace'
> 
> Thanks,
> 
> 	Ingo

I've been able to reproduce it, I'm looking at this right now. Thanks!

-Andrea
Andrea Righi Jan. 21, 2021, 10:15 a.m. UTC | #3
On Thu, Jan 21, 2021 at 09:52:01AM +0100, Andrea Righi wrote:
> On Thu, Jan 21, 2021 at 08:49:28AM +0100, Ingo Molnar wrote:
> > 
> > * tip-bot2 for Andrea Righi <tip-bot2@linutronix.de> wrote:
> > 
> > > The following commit has been merged into the x86/entry branch of tip:
> > > 
> > > Commit-ID:     e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> > > Gitweb:        https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> > > Author:        Andrea Righi <andrea.righi@canonical.com>
> > > AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
> > > Committer:     Ingo Molnar <mingo@kernel.org>
> > > CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00
> > > 
> > > x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
> > > 
> > > With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an
> > > empty object file.
> > > 
> > > With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU
> > > assembler doesn't generate a symbol table for empty object files and
> > > objtool fails with the following error when a valid symbol table cannot
> > > be found:
> > > 
> > >   arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table
> > > 
> > > To prevent this from happening, build thunk_$(BITS).o only if
> > > CONFIG_PREEMPTION is enabled.
> > > 
> > >   BugLink: https://bugs.launchpad.net/bugs/1911359
> > > 
> > > Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft")
> > > Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
> > > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > > Cc: Borislav Petkov <bp@alien8.de>
> > > Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390
> > 
> > Hm, this fails to build on UML defconfig:
> > 
> >  /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_thunk':
> >  /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:34: undefined reference to `preempt_schedule'
> >  /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_notrace_thunk':
> >  /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:35: undefined reference to `preempt_schedule_notrace'
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> I've been able to reproduce it, I'm looking at this right now. Thanks!

I see, basically UML selects ARCH_NO_PREEMPT, but in
arch/x86/um/Makefile it explicitly includes thunk_$(BITS).o regardless.

Considering that thunk_$(BITS) only contains preemption code now, we can
probably drop it from the Makefile, or, to be more consistent with the
x86 change, we could include it only if CONFIG_PREEMPTION is enabled
(even if it would never be, because UML has ARCH_NO_PREEMPT).

If it's unlikely that preemption will be enabled in UML one day I'd
probably go with the former, otherwise I'd go with the latter, because
it looks more consistent.

Opinions?

Thanks,
-Andrea

Patch
diff mbox series

diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
index 08bf95d..83c98da 100644
--- a/arch/x86/entry/Makefile
+++ b/arch/x86/entry/Makefile
@@ -21,12 +21,13 @@  CFLAGS_syscall_64.o		+= $(call cc-option,-Wno-override-init,)
 CFLAGS_syscall_32.o		+= $(call cc-option,-Wno-override-init,)
 CFLAGS_syscall_x32.o		+= $(call cc-option,-Wno-override-init,)
 
-obj-y				:= entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o
+obj-y				:= entry_$(BITS).o syscall_$(BITS).o
 obj-y				+= common.o
 
 obj-y				+= vdso/
 obj-y				+= vsyscall/
 
+obj-$(CONFIG_PREEMPTION)	+= thunk_$(BITS).o
 obj-$(CONFIG_IA32_EMULATION)	+= entry_64_compat.o syscall_32.o
 obj-$(CONFIG_X86_X32_ABI)	+= syscall_x32.o
 
diff --git a/arch/x86/entry/thunk_32.S b/arch/x86/entry/thunk_32.S
index f1f96d4..5997ec0 100644
--- a/arch/x86/entry/thunk_32.S
+++ b/arch/x86/entry/thunk_32.S
@@ -29,10 +29,8 @@  SYM_CODE_START_NOALIGN(\name)
 SYM_CODE_END(\name)
 	.endm
 
-#ifdef CONFIG_PREEMPTION
 	THUNK preempt_schedule_thunk, preempt_schedule
 	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
 	EXPORT_SYMBOL(preempt_schedule_thunk)
 	EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
-#endif
 
diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index 496b11e..9d543c4 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -31,14 +31,11 @@  SYM_FUNC_END(\name)
 	_ASM_NOKPROBE(\name)
 	.endm
 
-#ifdef CONFIG_PREEMPTION
 	THUNK preempt_schedule_thunk, preempt_schedule
 	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
 	EXPORT_SYMBOL(preempt_schedule_thunk)
 	EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
-#endif
 
-#ifdef CONFIG_PREEMPTION
 SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore)
 	popq %r11
 	popq %r10
@@ -53,4 +50,3 @@  SYM_CODE_START_LOCAL_NOALIGN(__thunk_restore)
 	ret
 	_ASM_NOKPROBE(__thunk_restore)
 SYM_CODE_END(__thunk_restore)
-#endif