Re: [PATCH] Revert "um: Enable CONFIG_CONSTRUCTORS"

On Wed, Dec 4, 2019 at 7:35 PM Anton Ivanov anton.ivanov@cambridgegreys.com wrote:

On 04/12/2019 16:43, Johannes Berg wrote:

...

From: Johannes Berg johannes.berg@intel.com

This reverts commit 786b2384bf1c ("um: Enable CONFIG_CONSTRUCTORS").

There are two issues with this commit, uncovered by Anton in tests on some (Debian) systems:

1. I completely forgot to call any constructors if CONFIG_CONSTRUCTORS isn't set. Don't recall now if it just wasn't needed on my system, or if I never tested this case.

2. With that fixed, it works - with CONFIG_CONSTRUCTORS *unset*. If I set CONFIG_CONSTRUCTORS, it fails again, which isn't totally unexpected since whatever wanted to run is likely to have to run before the kernel init etc. that calls the constructors in this case.

Basically, some constructors that gcc emits (libc has?) need to run very early during init; the failure mode otherwise was that the ptrace fork test already failed:

---

$ ./linux mem=512M Core dump limits : soft - 0 hard - NONE Checking that ptrace can change system call numbers...check_ptrace : child exited with exitcode 6, while expecting 0; status 0x67f Aborted

---

Thinking more about this, it's clear that we simply cannot support CONFIG_CONSTRUCTORS in UML. All the cases we need now (gcov, kasan) involve not use of the __attribute__((constructor)), but instead some constructor code/entry generated by gcc. Therefore, we cannot distinguish between kernel constructors and system constructors.

Thus, revert this commit.

Cc: stable@vger.kernel.org [5.4+] Fixes: 786b2384bf1c ("um: Enable CONFIG_CONSTRUCTORS") Reported-by: Anton Ivanov anton.ivanov@cambridgegreys.com Signed-off-by: Johannes Berg johannes.berg@intel.com

---

arch/um/include/asm/common.lds.S | 2 +- arch/um/kernel/dyn.lds.S | 1 + init/Kconfig | 1 + kernel/gcov/Kconfig | 2 +- 4 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/um/include/asm/common.lds.S b/arch/um/include/asm/common.lds.S index d7086b985f27..4049f2c46387 100644 --- a/arch/um/include/asm/common.lds.S +++ b/arch/um/include/asm/common.lds.S @@ -83,8 +83,8 @@ __preinit_array_end = .; } .init_array : {

•    /* dummy - we call this ourselves */
  __init_array_start = .;
  

• *(.init_array)
  __init_array_end = .;
  

  } .fini_array : {

diff --git a/arch/um/kernel/dyn.lds.S b/arch/um/kernel/dyn.lds.S index c69d69ee96be..f5001481010c 100644 --- a/arch/um/kernel/dyn.lds.S +++ b/arch/um/kernel/dyn.lds.S @@ -103,6 +103,7 @@ SECTIONS be empty, which isn't pretty. */ . = ALIGN(32 / 8); .preinit_array : { *(.preinit_array) }

• .init_array : { *(.init_array) } .fini_array : { *(.fini_array) } .data : { INIT_TASK_DATA(KERNEL_STACK_SIZE)

diff --git a/init/Kconfig b/init/Kconfig index b4daad2bac23..0328b53d09ad 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -54,6 +54,7 @@ config CC_DISABLE_WARN_MAYBE_UNINITIALIZED

config CONSTRUCTORS bool

• depends on !UML
  
  

  config IRQ_WORK bool

diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig index 060e8e726755..3941a9c48f83 100644 --- a/kernel/gcov/Kconfig +++ b/kernel/gcov/Kconfig @@ -4,7 +4,7 @@ menu "GCOV-based kernel profiling" config GCOV_KERNEL bool "Enable gcov-based kernel profiling" depends on DEBUG_FS

• select CONSTRUCTORS
  

• select CONSTRUCTORS if !UML
  default n
  ---help---
  This option enables gcov-based code profiling (e.g. for code coverage
  
  

Acked-by: Anton Ivanov anton.ivanov@cambridgegreys.co.uk

Applied. Thanks!