Re: [PATCH] zram: don't free statically defined names

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/09/23 11:02), Andrey Skvortsov wrote:
> The change is similar to that is used in comp_algorithm_set.
> This is detected by KASAN.
> 
> ==================================================================

---8<---

>  Unable to handle kernel paging request at virtual address ffffffffc1edc3c8
>  KASAN: maybe wild-memory-access in range
>  [0x0003fffe0f6e1e40-0x0003fffe0f6e1e47]
>  Mem abort info:
>    ESR = 0x0000000096000006
>    EC = 0x25: DABT (current EL), IL = 32 bits
>    SET = 0, FnV = 0
>    EA = 0, S1PTW = 0
>    FSC = 0x06: level 2 translation fault
>  Data abort info:
>    ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000
>    CM = 0, WnR = 0, TnD = 0, TagAccess = 0
>    GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>  swapper pgtable: 4k pages, 48-bit VAs, pgdp=00000000427dc000
>  [ffffffffc1edc3c8] pgd=00000000430e7003, p4d=00000000430e7003,
>  pud=00000000430e8003, pmd=0000000000000000
>  Internal error: Oops: 0000000096000006 [#1] SMP
> 
>  Tainted: [W]=WARN, [C]=CRAP, [N]=TEST
>  Hardware name: Pine64 PinePhone (1.2) (DT)
>  pstate: a0000005 (NzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>  pc : kfree+0x60/0x3a0
>  lr : zram_destroy_comps+0x98/0x198 [zram]
>  sp : ffff800089b57450
>  x29: ffff800089b57460 x28: 0000000000000004 x27: ffff800082833010
>  x26: 1fffe00000c8039c x25: 1fffe00000ba5004 x24: ffff000005d28000
>  x23: ffff800082533178 x22: ffff80007b71eaa8 x21: ffff000006401ce8
>  x20: ffff80007b70f7a0 x19: ffffffffc1edc3c0 x18: 1ffff00010506d6b
>  x17: 0000000000000000 x16: 0000000000000000 x15: ffff8000808e85e4
>  x14: ffff8000808e8478 x13: ffff80008003fa50 x12: ffff80008003f87c
>  x11: ffff800080011550 x10: ffff800081ee63f0 x9 : ffff80007b71eaa8
>  x8 : ffff80008003fa50 x7 : ffff80008003f87c x6 : 00000018a10e2f30
>  x5 : 00ffffffffffffff x4 : ffff00000ec93200 x3 : ffff00000bbee6e0
>  x2 : 0000000000000000 x1 : 0000000000000000 x0 : fffffdffc0000000

---8<---

The above is not needed in the commit message (not even sure if the
backtrace below is relevant).

>  Call trace:
>   kfree+0x60/0x3a0
>   zram_destroy_comps+0x98/0x198 [zram]
>   zram_reset_device+0x22c/0x4a8 [zram]
>   reset_store+0x1bc/0x2d8 [zram]
>   dev_attr_store+0x44/0x80
>   sysfs_kf_write+0xfc/0x188
>   kernfs_fop_write_iter+0x28c/0x428
>   vfs_write+0x4dc/0x9b8
>   ksys_write+0x100/0x1f8
>   __arm64_sys_write+0x74/0xb8
>   invoke_syscall+0xd8/0x260
>   el0_svc_common.constprop.0+0xb4/0x240
>   do_el0_svc+0x48/0x68
>   el0_svc+0x40/0xc8
>   el0t_64_sync_handler+0x120/0x130
>   el0t_64_sync+0x190/0x198

[..]

> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index c3d245617083d..d9d2c36658f59 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -2116,7 +2116,9 @@ static void zram_destroy_comps(struct zram *zram)
>  	}
>  
>  	for (prio = ZRAM_SECONDARY_COMP; prio < ZRAM_MAX_COMPS; prio++) {
> -		kfree(zram->comp_algs[prio]);
> +		/* Do not free statically defined compression algorithms */

We probably don't really need this comment.

> +		if (zram->comp_algs[prio] != default_compressor)
> +			kfree(zram->comp_algs[prio]);
>  		zram->comp_algs[prio] = NULL;
>  	}

OK, so... I wonder how do you get a `default_compressor` on a
non-ZRAM_PRIMARY_COMP prio.  May I ask what's your reproducer?

I didn't expect `default_compressor` on ZRAM_SECONDARY_COMP
and below.  As far as I can tell, we only do this:

	comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);

in zram_reset_device() and zram_add().  So, how does it end up in
ZRAM_SECONDARY_COMP...