[PATCH 3/5] zfs: Fix unaligned read of uint64

[mwleeds at mailtundra.com]

Hi Caleb,

Thanks for this interesting feedback. I saw these patches were already merged
since you sent this but I added a few thoughts below anyway.

On Friday, April 12th, 2024 at 11:50 AM, Caleb Connolly <caleb.connolly at linaro.org> wrote:

> Hi Phaedrus,
> 
> On 07/04/2024 03:47, mwleeds at mailtundra.com wrote:
> 
> > Without this patch, when trying to boot zfs using U-Boot on a Jetson TX2
> > NX (which is aarch64), I get a CPU reset error like so:
> > 
> > "Synchronous Abort" handler, esr 0x96000021
> > elr: 00000000800c9000 lr : 00000000800c8ffc (reloc)
> > elr: 00000000fff77000 lr : 00000000fff76ffc
> > x0 : 00000000ffb40f04 x1 : 0000000000000000
> > x2 : 000000000000000a x3 : 0000000003100000
> > x4 : 0000000003100000 x5 : 0000000000000034
> > x6 : 00000000fff9cc6e x7 : 000000000000000f
> > x8 : 00000000ff7f84a0 x9 : 0000000000000008
> > x10: 00000000ffb40f04 x11: 0000000000000006
> > x12: 000000000001869f x13: 0000000000000001
> > x14: 00000000ff7f84bc x15: 0000000000000010
> > x16: 0000000000002080 x17: 00000000001fffff
> > x18: 00000000ff7fbdd8 x19: 00000000ffb405f8
> > x20: 00000000ffb40dd0 x21: 00000000fffabe5e
> > x22: 000000ea77940000 x23: 00000000ffb42090
> > x24: 0000000000000000 x25: 0000000000000000
> > x26: 0000000000000000 x27: 0000000000000000
> > x28: 0000000000bab10c x29: 00000000ff7f85f0
> > 
> > Code: d00001a0 9103a000 94006ac6 f9401ba0 (f9400000)
> > Resetting CPU ...
> > 
> > This happens when be64_to_cpu() is called on a value that exists at a
> > memory address that's 4 byte aligned but not 8 byte aligned (e.g. an
> > address ending in 04). The call stack where that happens is:
> > check_pool_label() ->
> > zfs_nvlist_lookup_uint64(vdevnvlist, ZPOOL_CONFIG_ASHIFT,...) ->
> > be64_to_cpu()
> 
> 
> This is very odd, aarch64 doesn't generally have these restrictions. I
> got a bit nerdsniped when I saw this so I did some digging and figured this:
> 
> 1. Your abort exception doesn't include the FAR_ELx register (which
> should contain the address that was being accessed when the abort
> occured). This means your board is running in EL3.
> 2. It turns out there is an "A" flag in the SCTLR_ELx register, when set
> this flag causes a fault when trying to load from an address that isn't
> aligned to the size of the data element (see "A, bit" on
> https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/SCTLR-EL3--System-Control-Register--EL3-
> 
> I'm not sure who's in the "wrong" here, maybe the driver should avoid
> unaligned accesses? But then again, I don't think you should be running
> a ZFS driver in EL3.
> 
> I'm not familiar with the Jetson Nano, but maybe there's a documented
> way to run U-Boot so that it isn't executing in EL3? Or if not you could
> also try unsetting the A flag.

I may look into this if I get a chance. However if I write some assembly code
to change the execution level or unset the A flag, I worry that the code would
work fine on the hardware I have (Jetson TX2 NX) but behave differently on
another platform. And I don't think I can easily set up testing environments
with u-boot + zfs on different platforms to find out.

> 
> If this really is something to fix in the driver, I don't think
> hotpatching every unaligned access with a malloc() is the right solution.
> 

I'm certainly open to other ideas. The difficulty is the data structure we're
parsing in this file is read from disk and it's only 4-byte aligned.

> > Signed-off-by: Phaedrus Leeds mwleeds at mailtundra.com
> > Tested-by: Phaedrus Leeds mwleeds at mailtundra.com
> 
> 
> regarding your question about re-sending to remove these tags, I'd say
> probably yes, and especially if you're going to send a new revision anyway.
> 
> fwiw you seem to have gotten pretty much everything else about the patch
> submission process spot on :)
> 
> Kind regards,
> 
> > ---
> > fs/zfs/zfs.c | 18 ++++++++++++++++++
> > 1 file changed, 18 insertions(+)
> > 
> > diff --git a/fs/zfs/zfs.c b/fs/zfs/zfs.c
> > index 61d58fce68..9a50deac18 100644
> > --- a/fs/zfs/zfs.c
> > +++ b/fs/zfs/zfs.c
> > @@ -1552,35 +1552,53 @@ nvlist_find_value(char *nvlist, char *name, int valtype, char **val,
> > if (nelm_out)
> > *nelm_out = nelm;
> > return 1;
> > }
> > 
> > nvlist += encode_size; /* goto the next nvpair */
> > }
> > return 0;
> > }
> > 
> > +int is_word_aligned_ptr(void *ptr) {
> > + return ((uintptr_t)ptr & (sizeof(void *) - 1)) == 0;
> > +}
> > +
> > int
> > zfs_nvlist_lookup_uint64(char *nvlist, char *name, uint64_t *out)
> > {
> > char *nvpair;
> > size_t size;
> > int found;
> > 
> > found = nvlist_find_value(nvlist, name, DATA_TYPE_UINT64, &nvpair, &size, 0);
> > if (!found)
> > return 0;
> > if (size < sizeof(uint64_t)) {
> > printf("invalid uint64\n");
> > return ZFS_ERR_BAD_FS;
> > }
> > 
> > + /* On arm64, calling be64_to_cpu() on a value stored at a memory address
> > + * that's not 8-byte aligned causes the CPU to reset. Avoid that by copying the
> > + * value somewhere else if needed.
> > + */
> > + if (!is_word_aligned_ptr((void *)nvpair)) {
> > + uint64_t *alignedptr = malloc(sizeof(uint64_t));
> > + if (!alignedptr)
> > + return 0;
> > + memcpy(alignedptr, nvpair, sizeof(uint64_t));
> > + *out = be64_to_cpu(*alignedptr);
> > + free(alignedptr);
> > + return 1;
> > + }
> > +
> > out = be64_to_cpu((uint64_t *) nvpair);
> > return 1;
> > }
> > 
> > char *
> > zfs_nvlist_lookup_string(char *nvlist, char *name)
> > {
> > char *nvpair;
> > char *ret;
> > size_t slen;
> 
> 
> --
> // Caleb (they/them)