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    SubjectRe: [PATCH v3 4/4] slub: Force on no_hash_pointers when slub_debug is enabled
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    On 6/1/21 8:22 PM, Stephen Boyd wrote:
    > Obscuring the pointers that slub shows when debugging makes for some
    > confusing slub debug messages:
    >
    > Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17
    >
    > Those addresses are hashed for kernel security reasons. If we're trying
    > to be secure with slub_debug on the commandline we have some big
    > problems given that we dump whole chunks of kernel memory to the kernel
    > logs. Let's force on the no_hash_pointers commandline flag when
    > slub_debug is on the commandline. This makes slub debug messages more
    > meaningful and if by chance a kernel address is in some slub debug
    > object dump we will have a better chance of figuring out what went
    > wrong.
    >
    > Note that we don't use %px in the slub code because we want to reduce
    > the number of places that %px is used in the kernel. This also nicely
    > prints a big fat warning at kernel boot if slub_debug is on the
    > commandline so that we know that this kernel shouldn't be used on
    > production systems.
    >
    > Signed-off-by: Stephen Boyd <swboyd@chromium.org>

    Acked-by: Vlastimil Babka <vbabka@suse.cz>

    > ---
    > include/linux/kernel.h | 2 ++
    > lib/vsprintf.c | 2 +-
    > mm/slub.c | 4 ++++
    > 3 files changed, 7 insertions(+), 1 deletion(-)
    >
    > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
    > index 15d8bad3d2f2..bf950621febf 100644
    > --- a/include/linux/kernel.h
    > +++ b/include/linux/kernel.h
    > @@ -357,6 +357,8 @@ int sscanf(const char *, const char *, ...);
    > extern __scanf(2, 0)
    > int vsscanf(const char *, const char *, va_list);
    >
    > +extern int no_hash_pointers_enable(char *str);
    > +
    > extern int get_option(char **str, int *pint);
    > extern char *get_options(const char *str, int nints, int *ints);
    > extern unsigned long long memparse(const char *ptr, char **retptr);
    > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
    > index f0c35d9b65bf..cc281f5895f9 100644
    > --- a/lib/vsprintf.c
    > +++ b/lib/vsprintf.c
    > @@ -2186,7 +2186,7 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
    > bool no_hash_pointers __ro_after_init;
    > EXPORT_SYMBOL_GPL(no_hash_pointers);
    >
    > -static int __init no_hash_pointers_enable(char *str)
    > +int __init no_hash_pointers_enable(char *str)
    > {
    > if (no_hash_pointers)
    > return 0;
    > diff --git a/mm/slub.c b/mm/slub.c
    > index bf4949115412..a722794f1dbd 100644
    > --- a/mm/slub.c
    > +++ b/mm/slub.c
    > @@ -4460,6 +4460,10 @@ void __init kmem_cache_init(void)
    > if (debug_guardpage_minorder())
    > slub_max_order = 0;
    >
    > + /* Print slub debugging pointers without hashing */
    > + if (static_branch_unlikely(&slub_debug_enabled))
    > + no_hash_pointers_enable(NULL);
    > +
    > kmem_cache_node = &boot_kmem_cache_node;
    > kmem_cache = &boot_kmem_cache;
    >
    >

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