This commit addresses a potential memory leak in the
`get_mm_memcg_path()` function
by explicitly checking if the allocated buffer (`buf`) is NULL before
calling the
`css_put()` function. The prefix 'css' means abbreviation of cgroup_subsys_state

Previously, the code would directly call `css_put()` without checking
the value of
`buf`, which could lead to a memory leak if the buffer allocation failed.
This commit introduces a conditional check to ensure that `css_put()`
is only called
if `buf` is not NULL.

This change enhances the code's robustness and prevents memory leaks, improving
overall system stability.

**Specific Changes:**

* In the `out_put` label, an `if` statement is added to check
  if `buf` is not NULL before calling `css_put()`.

**Benefits:**

* Prevents potential memory leaks
* Enhances code robustness
* Improves system stability

Signed-off-by: Geunsik Lim <leemgs@gmail.com>
Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
---
 mm/mmap_lock.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 1854850b4b89..7314045b0e3b 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -213,7 +213,8 @@ static const char *get_mm_memcg_path(struct mm_struct *mm)
        cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);

 out_put:
-       css_put(&memcg->css);
+        if (buf != NULL)
+                css_put(&memcg->css);
 out:
        return buf;
 }
--
2.34.1
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On Mon, Jun 3, 2024 at 5:33 AM Geunsik Lim <geunsik.lim@gmail.com> wrote:
>
> This commit addresses a potential memory leak in the
> `get_mm_memcg_path()` function
> by explicitly checking if the allocated buffer (`buf`) is NULL before
> calling the
> `css_put()` function. The prefix 'css' means abbreviation of cgroup_subsys_state
>
> Previously, the code would directly call `css_put()` without checking
> the value of
> `buf`, which could lead to a memory leak if the buffer allocation failed.
> This commit introduces a conditional check to ensure that `css_put()`
> is only called
> if `buf` is not NULL.
>
> This change enhances the code's robustness and prevents memory leaks, improving
> overall system stability.
>
> **Specific Changes:**
>
> * In the `out_put` label, an `if` statement is added to check
>   if `buf` is not NULL before calling `css_put()`.
>
> **Benefits:**
>
> * Prevents potential memory leaks
> * Enhances code robustness
> * Improves system stability
>
> Signed-off-by: Geunsik Lim <leemgs@gmail.com>
> Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
> ---
>  mm/mmap_lock.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 1854850b4b89..7314045b0e3b 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -213,7 +213,8 @@ static const char *get_mm_memcg_path(struct mm_struct *mm)
>         cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);
>
>  out_put:
> -       css_put(&memcg->css);
> +        if (buf != NULL)
> +                css_put(&memcg->css);
>  out:
>         return buf;
>  }

I think the existing code is correct, and this change actually
introduces a memory leak where there was none before.

In the case where get_memcg_path_buf() returns NULL, we *still* need
to css_put() what we got from get_mem_cgroup_from_mm() before.

NAK, unless I'm missing something.

> --
> 2.34.1
> ----
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
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> Please read the FAQ at  http://www.tux.org/lkml/

On Mon, Jun 3, 2024 at 10:32 AM Axel Rasmussen <axelrasmussen@google.com> wrote:
>
> On Mon, Jun 3, 2024 at 5:33 AM Geunsik Lim <geunsik.lim@gmail.com> wrote:
> >
> > This commit addresses a potential memory leak in the
> > `get_mm_memcg_path()` function
> > by explicitly checking if the allocated buffer (`buf`) is NULL before
> > calling the
> > `css_put()` function. The prefix 'css' means abbreviation of cgroup_subsys_state
> >
> > Previously, the code would directly call `css_put()` without checking
> > the value of
> > `buf`, which could lead to a memory leak if the buffer allocation failed.
> > This commit introduces a conditional check to ensure that `css_put()`
> > is only called
> > if `buf` is not NULL.
> >
> > This change enhances the code's robustness and prevents memory leaks, improving
> > overall system stability.
> >
> > **Specific Changes:**
> >
> > * In the `out_put` label, an `if` statement is added to check
> >   if `buf` is not NULL before calling `css_put()`.
> >
> > **Benefits:**
> >
> > * Prevents potential memory leaks
> > * Enhances code robustness
> > * Improves system stability
> >
> > Signed-off-by: Geunsik Lim <leemgs@gmail.com>
> > Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
> > ---
> >  mm/mmap_lock.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> > index 1854850b4b89..7314045b0e3b 100644
> > --- a/mm/mmap_lock.c
> > +++ b/mm/mmap_lock.c
> > @@ -213,7 +213,8 @@ static const char *get_mm_memcg_path(struct mm_struct *mm)
> >         cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE);
> >
> >  out_put:
> > -       css_put(&memcg->css);
> > +        if (buf != NULL)
> > +                css_put(&memcg->css);
> >  out:
> >         return buf;
> >  }
>
> I think the existing code is correct, and this change actually
> introduces a memory leak where there was none before.
>
> In the case where get_memcg_path_buf() returns NULL, we *still* need
> to css_put() what we got from get_mem_cgroup_from_mm() before.
>
> NAK, unless I'm missing something.

+1

We already skip css_put() if get_mem_cgroup_from_mm() returns NULL.
Whether or not get_memcg_path_buf() succeeds is irrelevant here.

…
> Previously, the code would directly call `css_put()` without checking
> the value of
> `buf`, which could lead to a memory leak if the buffer allocation failed.
> This commit introduces a conditional check to ensure that `css_put()`
> is only called
> if `buf` is not NULL.
…

How did you come to the conclusion that such a source code adjustment
would trigger a desirable effect?
https://elixir.bootlin.com/linux/v6.10-rc2/source/mm/mmap_lock.c#L188

By the way:
Would you like to reconsider word wrapping for such change descriptions
any more?

Regards,
Markus