The word “limit” seems to be well defined. Looking in a number of online dictionaries, they all seem to agree on something like “the maximum or minimum allowable amount, quantity, or number”. That fits with my experience with things like
ATM withdrawal limits, speed limits, etc.
I think we’re setting the various $$Limit variables to the wrong values (or naming them wrong). If they were the actual limits, then these “-1”s wouldn’t be needed.
Chris
From: Kevin Peng <Kevin.Peng@arm.com>
Sent: January 27, 2023 10:38 PM
To: David Brown <david.brown@linaro.org>; Brand Chris (CSCA CSS ICW SW PSW 1) <Chris.Brand@infineon.com>
Cc: tf-m@lists.trustedfirmware.org
Subject: RE: [TF-M] Re: Region Limits
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Hi,
There was already a discussion on the SAU config issues that covers this discussion.
And a reminder to check SAU limit address settings:
The “-1” could be particular to SAU because SAU does a round-up on limit address.
That’s how it works. Please refer to the Arm v8m architecture specification or the examples in the above link.
Although it’s hard to define what is a “limit” because the limit address can be either included or excluded in different use cases, I do agree putting a “-1” in any region definitions could be confusing.
Like Bohdan suggested, we could move this “-1” to the SAU configuration codes.
And even provide some utility Macros for SAU region operations to ensure both the alignments “limits/boundaries” .
Thanks,
Kevin
From: David Brown via TF-M <tf-m@lists.trustedfirmware.org>
Sent: Friday, January 27, 2023 5:56 AM
To: Chris.Brand@infineon.com
Cc: tf-m@lists.trustedfirmware.org
Subject: [TF-M] Re: Region Limits
We have been seeing some issues with this specific value for some platforms. It also doesn't seem to handle the alignment properly, and I suspect the veneer region either ends up open to a lot more addresses than intended, or in other cases,
doesn't work at all.
I can't find good documentation on how the SAU regions work, but if it is anything like the MPU regions, the validity would use something like:
if ((address & ~align) == base)
I'm concerned that a lot of testing is happening on qemu, which does the range testing with < and >, which is not how it is implemented in the hardware (I think, as I said, I haven't found any
documentation on this).
But, yes, the limit needs to be the last valid address, regardless of alignment. Generally, the base needs to be a multiple of 1 greater than this value for the check to be meaningful.
David
On Thu, Jan 26, 2023 at 11:58 AM Chris.Brand--- via TF-M <tf-m@lists.trustedfirmware.org> wrote:
Hi,
Looking at platform/ext/common/tfm_hal_platform_v8m.c, this line stands out as quite odd:
.veneer_limit =
(uint32_t)®ION_NAME(Image$$, VENEER_ALIGN, $$Limit) - 1,
It seems that the various “$$Limit” values that exist within TF-M aren’t actually “limits” in the usual meaning of the word (the last value within the range) but are instead the first value *outside* the range. Elsewhere in TF-M we do seem to use the word “limit” in the conventional way.
I worry that this could lead to errors around the boundaries, particularly as some functions we use to check ranges take a “limit” parameter – we could easily see code that looks correct but that actual does the wrong thing, like
ARM_MPC_ConfigRegion(REGION_NAME(Image$$, ER_VENEER, $$Base),
REGION_NAME(Image$$, VENEER_ALIGN, $$Limit, attr);
Or a similar call to check_address_range().
For example, I suspect that this is a bug in the Nuvoton M2354 platform caused by this issue:
platform/ext/target/nuvoton/m2354/target_cfg.c:
.veneer_limit = (uint32_t)®ION_NAME(Image$$, VENEER_ALIGN, $$Limit),
It’s obviously a big thing to fix, but it does look to me to be worthwhile…
Chris Brand
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