Re: [PATCH V6 3/3] efi: Distinguish between "remaining space" andactually used space

[joeyli]

Hi all, 

æ äï2013-04-15 æ 13:09 -0700ïMatthew Garrett æåï
> EFI implementations distinguish between space that is actively used by a
> variable and space that merely hasn't been garbage collected yet. Space
> that hasn't yet been garbage collected isn't available for use and so isn't
> counted in the remaining_space field returned by QueryVariableInfo().
> 
> Combined with commit 68d9298 this can cause problems. Some implementations
> don't garbage collect until the remaining space is smaller than the maximum
> variable size, and as a result check_var_size() will always fail once more
> than 50% of the variable store has been used even if most of that space is
> marked as available for garbage collection. The user is unable to create
> new variables, and deleting variables doesn't increase the remaining space.
> 
> The problem that 68d9298 was attempting to avoid was one where certain
> platforms fail if the actively used space is greater than 50% of the
> available storage space. We should be able to calculate that by simply
> summing the size of each available variable and subtracting that from
> the total storage space. With luck this will fix the problem described in
> https://bugzilla.kernel.org/show_bug.cgi?id=55471 without permitting
> damage to occur to the machines 68d9298 was attempting to fix.
> 
> Signed-off-by: Matthew Garrett <matthew.garrett@xxxxxxxxxx>
> ---
>  arch/x86/platform/efi/efi.c | 109 +++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 102 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> index e844d82..a3f03cd 100644
> --- a/arch/x86/platform/efi/efi.c
> +++ b/arch/x86/platform/efi/efi.c
...
> @@ -1039,8 +1122,20 @@ efi_status_t efi_query_variable_store(u32 attributes, unsigned long size)
>  	if (status != EFI_SUCCESS)
>  		return status;
>  
> -	if (!storage_size || size > remaining_size || size > max_size ||
> -	    (remaining_size - size) < (storage_size / 2))
> +	/*
> +	 * Some firmware implementations refuse to boot if there's insufficient
> +	 * space in the variable store. We account for that by refusing the
> +	 * write if permitting it would reduce the available space to under
> +	 * 50%. However, some firmware won't reclaim variable space until
> +	 * after the used (not merely the actively used) space drops below
> +	 * a threshold. We can approximate that case with the value calculated
> +	 * above. If both the firmware and our calculations indicate that the
> +	 * available space would drop below 50%, refuse the write.
> +	 */
> +
> +	if (!storage_size || size > remaining_size ||
> +	    ((active_size + size + VAR_METADATA_SIZE > storage_size / 2) &&
> +	     (remaining_size - size < storage_size / 2)))

I am afraid it could not completely avoid to brick Samsung machines when
binding active_size and remaining_size logic with AND.

I don't have machine for testing, but my concern is we can run
delete/create variables many cycles at runtime for trigger brick Samsung
machines.
It causes the garbage size increased and remaining_size decreased. But
we still can create new variable because active_size doesn't increase
due to we delete variable before create new. So, the condition
"remaining_size - size < storage_size / 2" will not really hit because
active_size condition is pass.

And, here also can not use OR for binding active_size and remaining_size
logic, that causes many machines will not trigger garbage collection
because remaining_size will never tight.

Please let me know if I lost any other conditions or background
information.


Thanks a lot!
Joey Lee

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