tracing/histograms: Fix memory leak problem

In the Linux kernel, the following vulnerability has been resolved:

tracing/histograms: Fix memory leak problem

This reverts commit 46bbe5c671e06f070428b9be142cc4ee5cedebac.

As commit 46bbe5c671e0 ("tracing: fix double free") said, the

"double free" problem reported by clang static analyzer is:

> In parse_var_defs() if there is a problem allocating

> var_defs.expr, the earlier var_defs.name is freed.

> This free is duplicated by free_var_defs() which frees

> the rest of the list.

However, if there is a problem allocating N-th var_defs.expr:

+ in parse_var_defs(), the freed 'earlier var_defs.name' is

actually the N-th var_defs.name;

+ then in free_var_defs(), the names from 0th to (N-1)-th are freed;

IF ALLOCATING PROBLEM HAPPENED HERE!!! -+

\

|

0th 1th (N-1)-th N-th V

+-------------+-------------+-----+-------------+-----------

var_defs: | name | expr | name | expr | ... | name | expr | name | ///

+-------------+-------------+-----+-------------+-----------

These two frees don't act on same name, so there was no "double free"

problem before. Conversely, after that commit, we get a "memory leak"

problem because the above "N-th var_defs.name" is not freed.

If enable CONFIG_DEBUG_KMEMLEAK and inject a fault at where the N-th

var_defs.expr allocated, then execute on shell like:

$ echo 'hist:key=call_site:val=$v1,$v2:v1=bytes_req,v2=bytes_alloc' > \

/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

Then kmemleak reports:

unreferenced object 0xffff8fb100ef3518 (size 8):

comm "bash", pid 196, jiffies 4295681690 (age 28.538s)

hex dump (first 8 bytes):

76 31 00 00 b1 8f ff ff v1......

backtrace:

[<0000000038fe4895>] kstrdup+0x2d/0x60

[<00000000c99c049a>] event_hist_trigger_parse+0x206f/0x20e0

[<00000000ae70d2cc>] trigger_process_regex+0xc0/0x110

[<0000000066737a4c>] event_trigger_write+0x75/0xd0

[<000000007341e40c>] vfs_write+0xbb/0x2a0

[<0000000087fde4c2>] ksys_write+0x59/0xd0

[<00000000581e9cdf>] do_syscall_64+0x3a/0x80

[<00000000cf3b065c>] entry_SYSCALL_64_after_hwframe+0x46/0xb0