Re: [BUG] ext4 trace events cause NULL pointer dereferences

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hi Steven,

> create file: file_108.dat
> # sync
> (panic)
> 
> 
> Seems ac->ac_inode can be NULL:
> 
> DECLARE_EVENT_CLASS(ext4__mballoc,
> 	...
>         TP_fast_assign(
>                 __entry->dev            = ac->ac_inode->i_sb->s_dev;
>                 __entry->ino            = ac->ac_inode->i_ino;
> 		...
>         ),
> 	...
> );

Can you teach us proper tracepint writing way?


ext4_mb_release_group_pa() has a concern when ac is NULL.

============================================================
static noinline_for_stack int
ext4_mb_release_group_pa(struct ext4_buddy *e4b,
                                struct ext4_prealloc_space *pa,
                                struct ext4_allocation_context *ac)
{
        struct super_block *sb = e4b->bd_sb;
        ext4_group_t group;
        ext4_grpblk_t bit;

        trace_ext4_mb_release_group_pa(ac, pa);
        BUG_ON(pa->pa_deleted == 0);
        ext4_get_group_no_and_offset(sb, pa->pa_pstart, &group, &bit);
        BUG_ON(group != e4b->bd_group && pa->pa_len != 0);
        mb_free_blocks(pa->pa_inode, e4b, bit, pa->pa_len);
        atomic_add(pa->pa_len, &EXT4_SB(sb)->s_mb_discarded);

        if (ac) {                // here
                ac->ac_sb = sb;
                ac->ac_inode = NULL;
                ac->ac_b_ex.fe_group = group;
                ac->ac_b_ex.fe_start = bit;
                ac->ac_b_ex.fe_len = pa->pa_len;
                ac->ac_b_ex.fe_logical = 0;
                trace_ext4_mballoc_discard(ac);
        }

        return 0;
}
===================================================

but trace_ext4_mb_release_group_pa() doesn't care it.
=====================================================
TRACE_EVENT(ext4_mb_release_group_pa,
        TP_PROTO(struct ext4_allocation_context *ac,
                 struct ext4_prealloc_space *pa),

        TP_ARGS(ac, pa),

        TP_STRUCT__entry(
                __field(        dev_t,  dev                     )
                __field(        ino_t,  ino                     )
                __field(        __u64,  pa_pstart               )
                __field(        __u32,  pa_len                  )

        ),

        TP_fast_assign(
                __entry->dev            = ac->ac_sb->s_dev;
                __entry->ino            = ac->ac_inode->i_ino;
                __entry->pa_pstart      = pa->pa_pstart;
                __entry->pa_len         = pa->pa_len;
        ),

        TP_printk("dev %s pstart %llu len %u",
                  jbd2_dev_to_name(__entry->dev), __entry->pa_pstart, __entry->pa_len)
);
=====================================================

So, adding following branch should fix this issue.

        if (ac)
	        trace_ext4_mb_release_group_pa(ac, pa);

But, I don't think this is proper fix because we don't want any overhead
if the tracepoint is disabled.

So, How do we check NULL in TP_fast_assign()?

Thanks.