SLAB_MERGE_DEFAULT prevents definition of check_canary

[sempervictus]

When enabling SLAB_MERGE_DEFAULT, CONFIG_SLAB_CANARY cannot be defined, and check_canary depends on it. Since non ifdefd code references the check_canary function, this breaks the build:

mm/slub.c:1607:3: error: implicit declaration of function ‘check_canary’ [-Werror=implicit-function-declaration]
mm/slub.c:1607:28: error: ‘struct kmem_cache’ has no member named ‘random_active’
mm/slub.c:1623:3: error: implicit declaration of function ‘set_canary’; did you mean ‘set_capacity’? [-Werror=implicit-function-declaration]
mm/slub.c:1623:26: error: ‘struct kmem_cache’ has no member named ‘random_inactive’
mm/slub.c:1657:25: error: ‘struct kmem_cache’ has no member named ‘random_inactive’
mm/slub.c:2966:28: error: ‘struct kmem_cache’ has no member named ‘random_inactive’
mm/slub.c:2967:26: error: ‘struct kmem_cache’ has no member named ‘random_active’
mm/slub.c:3433:26: error: ‘struct kmem_cache’ has no member named ‘random_inactive’
mm/slub.c:3434:24: error: ‘struct kmem_cache’ has no member named ‘random_active’
mm/slub.c:3638:48: error: ‘struct kmem_cache’ has no member named ‘random_active’
mm/slub.c:4172:41: error: ‘struct kmem_cache’ has no member named ‘random_active’

[anthraxx]

You shouldn't use SLAB_MERGE_DEFAULT for a hardened kernel, but its obviously a bug when compilation breaks.

[anthraxx]

This is supposed to worked from the macro defined functions which should simply be optimized out all together. Which exact compiler and version of it are you using?

[sempervictus]

GCC 10.2 (Arch Linux). Thanks for digging into this

[anthraxx]

@sempervictus can you post your kconfig file, also did you do anything special or define specific flags? the defines must be in place when the real functions are not declared. I also couldn't make my POC fail whatever i did in a similar way.

[anthraxx]

@sempervictus let me make a guess: you do not have CONFIG_SLUB_DEBUG enabled, right?

[sempervictus]

@anthraxx: it is enabled:

$ grep CONFIG_SLUB_DEBUG config.sv 
CONFIG_SLUB_DEBUG=y
# CONFIG_SLUB_DEBUG_ON is not set

There's a bunch of other patches atop it, including UKSM which is why its being enabled in the config. The majority of the patch stack can be found here

[anthraxx]

please post your Kconfig

[anthraxx]

https://github.com/anthraxx/linux-hardened/commit/9642d52d1dfbf2de9a01cec6e4a42114693ea868