2022-06-30 01:37:31

by Zheng Yejian

[permalink] [raw]
Subject: [PATCH] Revert "tracing: fix double free"

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

Cc: [email protected]
Fixes: 46bbe5c671e0 ("tracing: fix double free")
Reported-by: Hulk Robot <[email protected]>
Signed-off-by: Zheng Yejian <[email protected]>
---
kernel/trace/trace_events_hist.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 48e82e141d54..2784951e0fc8 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -4430,6 +4430,7 @@ static int parse_var_defs(struct hist_trigger_data *hist_data)

s = kstrdup(field_str, GFP_KERNEL);
if (!s) {
+ kfree(hist_data->attrs->var_defs.name[n_vars]);
ret = -ENOMEM;
goto free;
}
--
2.32.0


2022-07-09 01:18:16

by Steven Rostedt

[permalink] [raw]
Subject: Re: [PATCH] Revert "tracing: fix double free"

On Thu, 30 Jun 2022 09:31:37 +0800
Zheng Yejian <[email protected]> wrote:

> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 48e82e141d54..2784951e0fc8 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -4430,6 +4430,7 @@ static int parse_var_defs(struct hist_trigger_data *hist_data)
>
> s = kstrdup(field_str, GFP_KERNEL);
> if (!s) {
> + kfree(hist_data->attrs->var_defs.name[n_vars]);

Instead of doing just a revert, can we also add, for safety:

hist_data->attrs->var_defs.name[n_vars] = NULL;

Thanks,

-- Steve

> ret = -ENOMEM;
> goto free;
> }

2022-07-11 02:01:53

by Zheng Yejian

[permalink] [raw]
Subject: [PATCH v2] 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

Cc: [email protected]
Fixes: 46bbe5c671e0 ("tracing: fix double free")
Reported-by: Hulk Robot <[email protected]>
Suggested-by: Steven Rostedt <[email protected]>
Signed-off-by: Zheng Yejian <[email protected]>
---
kernel/trace/trace_events_hist.c | 2 ++
1 file changed, 2 insertions(+)

Changes since v1:
- Assign 'NULL' after 'kfree' for safety as suggested by Steven
- Rename commit title and add Suggested-by tag

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 48e82e141d54..e87a46794079 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -4430,6 +4430,8 @@ static int parse_var_defs(struct hist_trigger_data *hist_data)

s = kstrdup(field_str, GFP_KERNEL);
if (!s) {
+ kfree(hist_data->attrs->var_defs.name[n_vars]);
+ hist_data->attrs->var_defs.name[n_vars] = NULL;
ret = -ENOMEM;
goto free;
}
--
2.32.0

2022-07-11 14:51:26

by Tom Zanussi

[permalink] [raw]
Subject: Re: [PATCH v2] tracing/histograms: Fix memory leak problem

Hi Yejian,

On 7/10/2022 8:47 PM, Zheng Yejian wrote:
> 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.

Good catch, thanks for fixing it.

So I'm wondering if this means that that the original unnecessary bugfix
was based on a bug in the clang static analyzer or if that would just be
considered a false positive...

Reviewed-by: Tom Zanussi <[email protected]>

Tom

>
> 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
>
> Cc: [email protected]
> Fixes: 46bbe5c671e0 ("tracing: fix double free")
> Reported-by: Hulk Robot <[email protected]>
> Suggested-by: Steven Rostedt <[email protected]>
> Signed-off-by: Zheng Yejian <[email protected]>
> ---
> kernel/trace/trace_events_hist.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> Changes since v1:
> - Assign 'NULL' after 'kfree' for safety as suggested by Steven
> - Rename commit title and add Suggested-by tag
>
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 48e82e141d54..e87a46794079 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -4430,6 +4430,8 @@ static int parse_var_defs(struct hist_trigger_data *hist_data)
>
> s = kstrdup(field_str, GFP_KERNEL);
> if (!s) {
> + kfree(hist_data->attrs->var_defs.name[n_vars]);
> + hist_data->attrs->var_defs.name[n_vars] = NULL;
> ret = -ENOMEM;
> goto free;
> }

2022-07-11 16:05:33

by Steven Rostedt

[permalink] [raw]
Subject: Re: [PATCH v2] tracing/histograms: Fix memory leak problem

On Mon, 11 Jul 2022 09:27:44 -0500
"Zanussi, Tom" <[email protected]> wrote:

> So I'm wondering if this means that that the original unnecessary bugfix
> was based on a bug in the clang static analyzer or if that would just be
> considered a false positive...

Good question. I'd like to know this, as if it is the case, I want to
report that in my pull request to Linus.

-- Steve

2022-07-12 11:53:31

by Zheng Yejian

[permalink] [raw]
Subject: Re: [PATCH v2] tracing/histograms: Fix memory leak problem

On Mon, 11 Jul 2022 11:52:11 -0400
Steven Rostedt <[email protected]> wrote:
> On Mon, 11 Jul 2022 09:27:44 -0500
> "Zanussi, Tom" <[email protected]> wrote:
>
> > So I'm wondering if this means that that the original unnecessary bugfix
> > was based on a bug in the clang static analyzer or if that would just be
> > considered a false positive...
>
> Good question. I'd like to know this, as if it is the case, I want to
> report that in my pull request to Linus.
>
> -- Steve

I didn't use clang static analyzer before, but from its home page,
'False Positives' seems to exist, see https://clang-analyzer.llvm.org/:
> Static analysis is not perfect. It can falsely flag bugs in a program
> where the code behaves correctly. Because some code checks require more
> analysis precision than others, the frequency of false positives can
> vary widely between different checks. Our long-term goal is to have the
> analyzer have a low false positive rate for most code on all checks.

So I try the clang-14 which comes from
https://github.com/llvm/llvm-project/releases/tag/llvmorg-14.0.0,
then execute like:
$ scan-build make -j16

Then I take a rough look at following warnings related to 'trace_events_hist.c'
(serial number is manually added, no double free warning, maybe due to clang version):
1. kernel/trace/trace_events_hist.c:1540:6: warning: Branch condition evaluates to a garbage value [core.uninitialized.Branch]
if (!attrs->keys_str) {
^~~~~~~~~~~~~~~~
2. kernel/trace/trace_events_hist.c:1580:9: warning: Array access (via field 'field_var_str') results in a null pointer dereference [core.NullDereference]
kfree(elt_data->field_var_str[i]);
^~~~~~~~~~~~~~~~~~~~~~~~~~
3. kernel/trace/trace_events_hist.c:1898:3: warning: 1st function call argument is an uninitialized value [core.CallAndMessage]
destroy_hist_field(hist_field->operands[i], level + 1);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4. kernel/trace/trace_events_hist.c:2095:2: warning: 1st function call argument is an uninitialized value [core.CallAndMessage]
kfree(ref_field->system);
^~~~~~~~~~~~~~~~~~~~~~~~
5. kernel/trace/trace_events_hist.c:2099:2: warning: 1st function call argument is an uninitialized value [core.CallAndMessage]
kfree(ref_field->name);
^~~~~~~~~~~~~~~~~~~~~~
6. kernel/trace/trace_events_hist.c:2158:4: warning: Potential leak of memory pointed to by 'ref_field' [unix.Malloc]
return NULL;

Since I'm not very familiar with trace_events_hist.c, I roughly conclude that:
1. warning 1/3/6 are plausible but false-positive;
2. warning 2/4/5 seems positive although they don't cause practical problems because
elt_data->field_var_str[i] / ref_field->system / ref_field->name can be 'NULL'
on 'kfree'. Do we need to explicitly check 'NULL' there?

2022-07-12 15:01:06

by Steven Rostedt

[permalink] [raw]
Subject: Re: [PATCH v2] tracing/histograms: Fix memory leak problem

On Tue, 12 Jul 2022 19:48:44 +0800
Zheng Yejian <[email protected]> wrote:

> Since I'm not very familiar with trace_events_hist.c, I roughly conclude that:
> 1. warning 1/3/6 are plausible but false-positive;
> 2. warning 2/4/5 seems positive although they don't cause practical problems because
> elt_data->field_var_str[i] / ref_field->system / ref_field->name can be 'NULL'
> on 'kfree'. Do we need to explicitly check 'NULL' there?

It's confusing code. Both Tom and I missed it as well.

-- Steve