Re: [PATCH v2] ACPI: GTDT: simplify acpi_gtdt_init() implementation

[Lorenzo Pieralisi]

On Wed, Oct 09, 2024 at 12:33:35PM +0100, Marc Zyngier wrote:
> On Tue, 08 Oct 2024 15:04:52 +0100,
> Zheng Zengkai <zhengzengkai@xxxxxxxxxx> wrote:
> > 
> > 
> > 在 2024/10/8 16:55, Marc Zyngier 写道:
> > > On Tue, 08 Oct 2024 09:24:29 +0100,
> > > Zheng Zengkai <zhengzengkai@xxxxxxxxxx> wrote:
> > >> According to GTDT Table Structure of ACPI specification, the result of
> > >> expression '(void *)gtdt + gtdt->platform_timer_offset' will be same
> > >> with the expression '(void *)table + sizeof(struct acpi_table_gtdt)'
> > > There is no such language in the spec. It simply says "Offset to the
> > > Platform Timer Structure[] array from the start of this table".
> > OK, I mean that in current code, the condition of this check is redundant.
> 
> That's not my reading if it. Where do you see another validity check
> that makes this one superfluous?
> 
> > >> in function acpi_gtdt_init(), so the condition of the "invalid timer
> > >> data" check will never be true, remove the EINVAL error check branch
> > >> and change to void return type for acpi_gtdt_init() to simplify the
> > >> function implementation and error handling by callers.
> > > And ACPI tables are well known to be always correct, right?
> > Not always, check is needed, but should be changed.
> 
> You are not changing it, you are getting rid of it, and I don't see
> you replacing it with anything else.
> 
> > >> Besides, after commit c2743a36765d ("clocksource: arm_arch_timer: add
> > >> GTDT support for memory-mapped timer"), acpi_gtdt_init() currently will
> > >> not be called with parameter platform_timer_count set to NULL and we
> > >> can explicitly initialize the integer variable which is used for storing
> > >> the number of platform timers by caller to zero, so there is no need to
> > >> do null pointer check for platform_timer_count in acpi_gtdt_init(),
> > >> remove it to make code a bit more concise.
> > >> 
> > >> Signed-off-by: Zheng Zengkai <zhengzengkai@xxxxxxxxxx>
> > >> ---
> > >> Changes in v2:
> > >> - initialize 'ret' in gtdt_sbsa_gwdt_init() to silence build warning
> > >> 
> > >> v1: https://lore.kernel.org/all/20240930030716.179992-1-zhengzengkai@xxxxxxxxxx/
> > >> ---
> > >>   drivers/acpi/arm64/gtdt.c            | 31 +++++++---------------------
> > >>   drivers/clocksource/arm_arch_timer.c |  6 ++----
> > >>   include/linux/acpi.h                 |  2 +-
> > >>   3 files changed, 11 insertions(+), 28 deletions(-)
> > >> 
> > >> diff --git a/drivers/acpi/arm64/gtdt.c b/drivers/acpi/arm64/gtdt.c
> > >> index c0e77c1c8e09..7fe27c0edde7 100644
> > >> --- a/drivers/acpi/arm64/gtdt.c
> > >> +++ b/drivers/acpi/arm64/gtdt.c
> > >> @@ -147,45 +147,30 @@ bool __init acpi_gtdt_c3stop(int type)
> > >>    * @table:			The pointer to GTDT table.
> > >>    * @platform_timer_count:	It points to a integer variable which is used
> > >>    *				for storing the number of platform timers.
> > >> - *				This pointer could be NULL, if the caller
> > >> - *				doesn't need this info.
> > >> - *
> > >> - * Return: 0 if success, -EINVAL if error.
> > >>    */
> > >> -int __init acpi_gtdt_init(struct acpi_table_header *table,
> > >> +void __init acpi_gtdt_init(struct acpi_table_header *table,
> > >>   			  int *platform_timer_count)
> > >>   {
> > >> -	void *platform_timer;
> > >>   	struct acpi_table_gtdt *gtdt;
> > >>     	gtdt = container_of(table, struct acpi_table_gtdt, header);
> > >>   	acpi_gtdt_desc.gtdt = gtdt;
> > >>   	acpi_gtdt_desc.gtdt_end = (void *)table + table->length;
> > >>   	acpi_gtdt_desc.platform_timer = NULL;
> > >> -	if (platform_timer_count)
> > >> -		*platform_timer_count = 0;
> > >>     	if (table->revision < 2) {
> > >>   		pr_warn("Revision:%d doesn't support Platform Timers.\n",
> > >>   			table->revision);
> > >> -		return 0;
> > >> +		return;
> > >>   	}
> > >>     	if (!gtdt->platform_timer_count) {
> > >>   		pr_debug("No Platform Timer.\n");
> > >> -		return 0;
> > >> +		return;
> > >>   	}
> > >>   -	platform_timer = (void *)gtdt + gtdt->platform_timer_offset;
> > >> -	if (platform_timer < (void *)table + sizeof(struct acpi_table_gtdt)) {
> > >> -		pr_err(FW_BUG "invalid timer data.\n");
> > >> -		return -EINVAL;
> > >> -	}
> > >> -	acpi_gtdt_desc.platform_timer = platform_timer;
> > >> -	if (platform_timer_count)
> > >> -		*platform_timer_count = gtdt->platform_timer_count;
> > >> -
> > >> -	return 0;
> > >> +	acpi_gtdt_desc.platform_timer = (void *)gtdt + gtdt->platform_timer_offset;
> > > And now you are trusting something that potentially points to some
> > > unexpected location, blindly using it. It is bad enough that the
> > > current checks are pretty poor (no check against the end of the
> > > table for the first timer entry), but you are making it worse.
> > > 
> > > 	M.
> > 
> > Can I use the second and third bytes (the length) of platform timer
> > structure to check against the end of the table ?
> 
> That's how it is supposed to be done indeed.

AFAICS I think first we need to check whether the platform_timer pointer
is within gtdt bounds (< gtdt_end) before de-referencing what it points
at to detect the (first) GT entry length and check that it is within
gtdt_end too. We do that only in next_platform_timer() for subsequent
GT blocks.

I agree with Marc, current check is fine, we should add to it, not
remove it.

Thanks,
Lorenzo