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    SubjectRe: [PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG
    On Mon, 23 Apr 2018, Mikulas Patocka wrote:

    > The kvmalloc function tries to use kmalloc and falls back to vmalloc if
    > kmalloc fails.
    >
    > Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
    > uses DMA-API on the returned memory or frees it with kfree. Such bugs were
    > found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
    > code.
    >
    > These bugs are hard to reproduce because kvmalloc falls back to vmalloc
    > only if memory is fragmented.
    >
    > In order to detect these bugs reliably I submit this patch that changes
    > kvmalloc to fall back to vmalloc with 1/2 probability if CONFIG_DEBUG_SG
    > is turned on. CONFIG_DEBUG_SG is used, because it makes the DMA API layer
    > verify the addresses passed to it, and so the user will get a reliable
    > stacktrace.
    >

    Why not just do it unconditionally? Sounds better than "50% of the time
    this will catch bugs".

    > Some bugs (such as buffer overflows) are better detected
    > with kmalloc code, so we must test the kmalloc path too.
    >
    > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    >
    > ---
    > mm/util.c | 10 ++++++++++
    > 1 file changed, 10 insertions(+)
    >
    > Index: linux-2.6/mm/util.c
    > ===================================================================
    > --- linux-2.6.orig/mm/util.c 2018-04-23 00:12:05.000000000 +0200
    > +++ linux-2.6/mm/util.c 2018-04-23 17:57:02.000000000 +0200
    > @@ -14,6 +14,7 @@
    > #include <linux/hugetlb.h>
    > #include <linux/vmalloc.h>
    > #include <linux/userfaultfd_k.h>
    > +#include <linux/random.h>
    >
    > #include <asm/sections.h>
    > #include <linux/uaccess.h>
    > @@ -404,6 +405,12 @@ void *kvmalloc_node(size_t size, gfp_t f
    > */
    > WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
    >
    > +#ifdef CONFIG_DEBUG_SG
    > + /* Catch bugs when the caller uses DMA API on the result of kvmalloc. */
    > + if (!(prandom_u32_max(2) & 1))
    > + goto do_vmalloc;
    > +#endif
    > +
    > /*
    > * We want to attempt a large physically contiguous block first because
    > * it is less likely to fragment multiple larger blocks and therefore
    > @@ -427,6 +434,9 @@ void *kvmalloc_node(size_t size, gfp_t f
    > if (ret || size <= PAGE_SIZE)
    > return ret;
    >
    > +#ifdef CONFIG_DEBUG_SG
    > +do_vmalloc:
    > +#endif

    You can just do

    do_vmalloc: __maybe_unused

    > return __vmalloc_node_flags_caller(size, node, flags,
    > __builtin_return_address(0));
    > }
    >
    >

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