Re: (BTF) [PATCH] mm/page_alloc: Work around a pahole limitation with zero-sized struct pagesets

[Michal Suchánek]

On Wed, May 26, 2021 at 09:07:41AM +0100, Mel Gorman wrote:
> Michal Suchanek reported the following problem with linux-next
> 
>   [    0.000000] Linux version 5.13.0-rc2-next-20210519-1.g3455ff8-vanilla (geeko@buildhost) (gcc (SUSE Linux) 10.3.0, GNU ld (GNU Binutils; openSUSE Tumbleweed) 2.36.1.20210326-3) #1 SMP Wed May 19 10:05:10 UTC 2021 (3455ff8)
>   [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.13.0-rc2-next-20210519-1.g3455ff8-vanilla root=UUID=ec42c33e-a2c2-4c61-afcc-93e9527 8f687 plymouth.enable=0 resume=/dev/disk/by-uuid/f1fe4560-a801-4faf-a638-834c407027c7 mitigations=auto earlyprintk initcall_debug nomodeset earlycon ignore_loglevel console=ttyS0,115200
> ...
>   [   26.093364] calling  tracing_set_default_clock+0x0/0x62 @ 1
>   [   26.098937] initcall tracing_set_default_clock+0x0/0x62 returned 0 after 0 usecs
>   [   26.106330] calling  acpi_gpio_handle_deferred_request_irqs+0x0/0x7c @ 1
>   [   26.113033] initcall acpi_gpio_handle_deferred_request_irqs+0x0/0x7c returned 0 after 3 usecs
>   [   26.121559] calling  clk_disable_unused+0x0/0x102 @ 1
>   [   26.126620] initcall clk_disable_unused+0x0/0x102 returned 0 after 0 usecs
>   [   26.133491] calling  regulator_init_complete+0x0/0x25 @ 1
>   [   26.138890] initcall regulator_init_complete+0x0/0x25 returned 0 after 0 usecs
>   [   26.147816] Freeing unused decrypted memory: 2036K
>   [   26.153682] Freeing unused kernel image (initmem) memory: 2308K
>   [   26.165776] Write protecting the kernel read-only data: 26624k
>   [   26.173067] Freeing unused kernel image (text/rodata gap) memory: 2036K
>   [   26.180416] Freeing unused kernel image (rodata/data gap) memory: 1184K
>   [   26.187031] Run /init as init process
>   [   26.190693]   with arguments:
>   [   26.193661]     /init
>   [   26.195933]   with environment:
>   [   26.199079]     HOME=/
>   [   26.201444]     TERM=linux
>   [   26.204152]     BOOT_IMAGE=/boot/vmlinuz-5.13.0-rc2-next-20210519-1.g3455ff8-vanilla
>   [   26.254154] BPF:      type_id=35503 offset=178440 size=4
>   [   26.259125] BPF:
>   [   26.261054] BPF:Invalid offset
>   [   26.264119] BPF:
>   [   26.264119]
>   [   26.267437] failed to validate module [efivarfs] BTF: -22
> 
> Andrii Nakryiko bisected the problem to the commit "mm/page_alloc: convert
> per-cpu list protection to local_lock" currently staged in mmotm. In his
> own words
> 
>   The immediate problem is two different definitions of numa_node per-cpu
>   variable. They both are at the same offset within .data..percpu ELF
>   section, they both have the same name, but one of them is marked as
>   static and another as global. And one is int variable, while another
>   is struct pagesets. I'll look some more tomorrow, but adding Jiri and
>   Arnaldo for visibility.
> 
>   [110907] DATASEC '.data..percpu' size=178904 vlen=303
>   ...
>         type_id=27753 offset=163976 size=4 (VAR 'numa_node')
>         type_id=27754 offset=163976 size=4 (VAR 'numa_node')
> 
>   [27753] VAR 'numa_node' type_id=27556, linkage=static
>   [27754] VAR 'numa_node' type_id=20, linkage=global
> 
>   [20] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> 
>   [27556] STRUCT 'pagesets' size=0 vlen=1
>         'lock' type_id=507 bits_offset=0
> 
>   [506] STRUCT '(anon)' size=0 vlen=0
>   [507] TYPEDEF 'local_lock_t' type_id=506
> 
> The patch in question introduces a zero-sized per-cpu struct and while
> this is not wrong, versions of pahole prior to 1.22 (unreleased) get
> confused during BTF generation with two separate variables occupying the
> same address.
> 
> This patch checks for older versions of pahole and forces struct pagesets
> to be non-zero sized as a workaround when CONFIG_DEBUG_INFO_BTF is set. A
> warning is omitted so that distributions can update pahole when 1.22
> is released.
> 
> Reported-by: Michal Suchanek <msuchanek@xxxxxxx>
> Reported-by: Hritik Vijay <hritikxx8@xxxxxxxxx>
> Debugged-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  lib/Kconfig.debug |  3 +++
>  mm/page_alloc.c   | 11 +++++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 678c13967580..f88a155b80a9 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -313,6 +313,9 @@ config DEBUG_INFO_BTF
>  config PAHOLE_HAS_SPLIT_BTF
>  	def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "119")
>  
> +config PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT
> +	def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "122")
> +

This does not seem workable with dummy-tools.

Do we even have dummy pahole?

Thanks

Michal

>  config DEBUG_INFO_BTF_MODULES
>  	def_bool y
>  	depends on DEBUG_INFO_BTF && MODULES && PAHOLE_HAS_SPLIT_BTF
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ff8f706839ea..1d56d3de8e08 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -124,6 +124,17 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
>  
>  struct pagesets {
>  	local_lock_t lock;
> +#if defined(CONFIG_DEBUG_INFO_BTF) &&			\
> +    !defined(CONFIG_DEBUG_LOCK_ALLOC) &&		\
> +    !defined(CONFIG_PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT)
> +	/*
> +	 * pahole 1.21 and earlier gets confused by zero-sized per-CPU
> +	 * variables and produces invalid BTF. Ensure that
> +	 * sizeof(struct pagesets) != 0 for older versions of pahole.
> +	 */
> +	char __pahole_hack;
> +	#warning "pahole too old to support zero-sized struct pagesets"
> +#endif
>  };
>  static DEFINE_PER_CPU(struct pagesets, pagesets) = {
>  	.lock = INIT_LOCAL_LOCK(lock),