Re: malloc() alignment on 32 bit

[Sascha Hauer <sha@pengutronix.de>]

On Mon, Sep 19, 2022 at 04:24:19PM +0200, Enrico Scholz wrote:
> Sascha Hauer <sha@pengutronix.de> writes:
> 
> >> | zstd_decomp_init:536 workspace=8ff1a004+161320
> >> | ERROR: initcall ubifs_init+0x1/0xc4 failed: Invalid argument
> >> 
> > If you had asked me which alignment we have then I would have said it's
> > bigger. OTOH I never received any reports about insufficient alignment
> > on ARM or any other 32bit architecture.
> 
> The code which failed for me was added 3 months ago
> 
> | commit b4a9782d4f56333e897dccc35c2c27e2605f6b93
> | Author: Ahmad Fatoum <a.fatoum@pengutronix.de>
> | Date:   Wed Jul 13 12:09:18 2022 +0200
> | 
> |     lib: zstd: sync with Linux
> 
> and the Kconfig options (FS_UBIFS_COMPRESSION_ZSTD) are set to "off"...

When I said I received no reports about insufficient malloc alignment I
meant reports about erroneous accesses, like wrong data read/writes or data
aborts.

> 
> 
> > I suspect we could just drop the check without any harm, but that's just
> > a gut feeling because we never had any alignment issues.
> >
> > BTW are you sure ldrd/strd need 8 byte alignment?
> 
> Their EX variants (LDREXD + STREXD; see [1]).  Unaligned access on
> plain LDRD/STRD is allowed on ARMv7-A.  But not on ARMv7-M or ARMv6 and
> earlier.

Ok, I found this for the LDRD instruction:

| Prior to ARMv6, if the memory address is not 64-bit aligned, the data read from memory is
| UNPREDICTABLE. Alignment checking (taking a data abort), and support for a big-endian
| (BE-32) data format are implementation options.

So it seems it's really a good idea to increase malloc alignment
accordingly.

Sascha

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