Re: [PATCH 1/1] s390/crash: allow multi-segment iterators

Alexander Egorenkov
2022-07-06T08:18:37Z

Hi Alexander,

Alexander Gordeev <agordeev@linux.ibm.com> writes:

> Rework copy_oldmem_page() to allow multi-segment iterators.
> Reuse existing iterate_iovec macro as is and only relevant
> bits from __iterate_and_advance macro.
>
> Fixes: 49b11524d648 ("s390/crash: add missing iterator advance in copy_oldmem_page())
> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
> ---
> arch/s390/kernel/crash_dump.c | 65 +++++++++++++++++++++++++++--------
> 1 file changed, 50 insertions(+), 15 deletions(-)
>
> diff --git a/arch/s390/kernel/crash_dump.c b/arch/s390/kernel/crash_dump.c
> index 28124d0fa1d5..ac873245d6f0 100644
> --- a/arch/s390/kernel/crash_dump.c
> +++ b/arch/s390/kernel/crash_dump.c
> @@ -210,6 +210,52 @@ static int copy_oldmem_user(void __user *dst, unsigned long src, size_t count)
> return 0;
> }
>
> +#define iterate_iovec(i, n, base, len, off, __p, STEP) { \
> + size_t off = 0; \
> + size_t skip = i->iov_offset; \
> + do { \
> + len = min(n, __p->iov_len - skip); \
> + if (likely(len)) { \
> + base = __p->iov_base + skip; \
> + len -= (STEP); \
> + off += len; \
> + skip += len; \
> + n -= len; \
> + if (skip < __p->iov_len) \
> + break; \
> + } \
> + __p++; \
> + skip = 0; \
> + } while (n); \
> + i->iov_offset = skip; \
> + n = off; \
> +}
> +
> +#define __iterate_and_advance(i, n, base, len, off, I, K) { \
> + if (unlikely(i->count < n)) \
> + n = i->count; \
> + if (likely(n)) { \
> + if (likely(iter_is_iovec(i))) { \
> + const struct iovec *iov = i->iov; \
> + void __user *base; \
> + size_t len; \
> + iterate_iovec(i, n, base, len, off, \
> + iov, (I)) \
> + i->nr_segs -= iov - i->iov; \
> + i->iov = iov; \
> + } else if (iov_iter_is_kvec(i)) { \
> + const struct kvec *kvec = i->kvec; \
> + void *base; \
> + size_t len; \
> + iterate_iovec(i, n, base, len, off, \
> + kvec, (K)) \
> + i->nr_segs -= kvec - i->kvec; \
> + i->kvec = kvec; \
> + } \
> + i->count -= n; \
> + } \
> +}
> +
> /*
> * Copy one page from "oldmem"
> */
> @@ -217,25 +263,14 @@ ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned long pfn, size_t csize,
> unsigned long offset)
> {
> unsigned long src;
> - int rc;
>
> if (!(iter_is_iovec(iter) || iov_iter_is_kvec(iter)))
> return -EINVAL;
> - /* Multi-segment iterators are not supported */
> - if (iter->nr_segs > 1)
> - return -EINVAL;
> - if (!csize)
> - return 0;
> src = pfn_to_phys(pfn) + offset;
> -
> - /* XXX: pass the iov_iter down to a common function */
> - if (iter_is_iovec(iter))
> - rc = copy_oldmem_user(iter->iov->iov_base, src, csize);
> - else
> - rc = copy_oldmem_kernel(iter->kvec->iov_base, src, csize);
> - if (rc < 0)
> - return rc;
> - iov_iter_advance(iter, csize);
> + __iterate_and_advance(iter, csize, base, len, off,
> + ({ copy_oldmem_user(base, src + off, len) < 0 ? csize : 0; }),
> + ({ copy_oldmem_kernel(base, src + off, len) < 0 ? csize : 0; })

Question
--------
About return value of STEP in iterate_iovec().
We return "csize" in case copy_oldmem_*() fails.
If i'm not mistaken this could lead to an overflow in iterate_iovec():

len -= (STEP);

Because len could be less than csize in case iovec consists of multiple
segments, one of which is less than csize.

Better to return len ?

({ copy_oldmem_user(base, src + off, len) < 0 ? len : 0; })

> + )
> return csize;
> }

Another thing is that now we never report any errors in contrast to
the version before. This is OK ?
Maybe set an error flag while iterating and then when the iteration is
done, check the flag and return an error ?

>
> --
> 2.34.1

Otherwise, looks good to me.
Tested on LPAR and zVM , all our tela-kernel kdump tests in
tests/dump/kdump work now.

Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>

Regards
Alex