[PATCH 06/17] fs/squashfs: sqfs_read_directory_table: fix memory leak

Richard Genoud
2020-10-15T16:29:45Z

Hi Miquel !
Thanks for your feedback.

Le 15/10/2020 ? 15:54, Miquel Raynal a ?crit?:
> Hi Richard,
>
> Richard Genoud <richard.genoud@posteo.net> wrote on Wed, 14 Oct 2020
> 10:06:11 +0200:
>
>> pos_list wasn't freed on every error
>>
>> Signed-off-by: Richard Genoud <richard.genoud@posteo.net>
>
> Same comment here (and probably after as well) as in patch 05/17, not
> sure this is actually relevant for the community but I prefer this:
>
> bar = malloc();
> ...
> if (ret)
> goto free_bar;
>
> foo = malloc();
> ...
> if (ret)
> goto free foo;
>
> ...
>
> foo:
> kfree(foo);
> bar:
> kfree(bar);
>
> than:
>
> foo = NULL;
> bar = NULL;
>
> ...
> if (ret)
> goto out;
> ...
> if (ret)
> goto out;
> ...
> out:
> if (ret)
> kfree(...)

I guess it's a coding habit.
I personnaly prefer the later because I think it's less error-prone :
When moving code aroung, we don't have to move the labels and rename
the gotos.
Ex:
Let's say we have this code:
bar = malloc();
...
if (ret)
goto free_bar;

foo = malloc();
...
if (ret)
goto free_foo;
ret = init_somthing();
if (ret)
goto free_foo;
ret = dummy()
if (ret)
goto free_foo;

...

foo:
kfree(foo);
bar:
kfree(bar);

And, we want to move, for whatever reason, init_something() and dummy()
before the foo allocation. We will have to change the code to:

bar = malloc();
...
if (ret)
goto free_bar;
ret = init_somthing();
if (ret)
goto free_bar; // not free_foo anymore !
ret = dummy()
if (ret)
goto free_bar; // ditto

foo = malloc();
...
if (ret)
goto free_foo;
...

foo:
kfree(foo);
bar:
kfree(bar);

Worse, if we have to exchange bar and foo allocation, we'll also have
to exchange the deallocation of foo and bar and change all gotos beneath :
foo = malloc();
...
if (ret)
goto free_foo;

bar = malloc();
...
if (ret)
goto free_bar;

ret = init_somthing();
if (ret)
goto free_foo; // not free_foo anymore
ret = dummy()
if (ret)
goto free_foo; //ditto

...

// oops ! we have to exchange that !
foo:
kfree(foo);
bar:
kfree(bar);

That's why I prefer only one label and setting NULL.
If I didn't convince you, I'll change it back to multiple labels :)

>
>> ---
>> fs/squashfs/sqfs.c | 31 +++++++++++++++++--------------
>> 1 file changed, 17 insertions(+), 14 deletions(-)
>>
>> diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
>> index 55d183663a8..c4d74fd4d6d 100644
>> --- a/fs/squashfs/sqfs.c
>> +++ b/fs/squashfs/sqfs.c
>> @@ -722,6 +722,8 @@ static int sqfs_read_directory_table(unsigned char **dir_table, u32 **pos_list)
>> unsigned long dest_len = 0;
>> bool compressed;
>>
>> + *dir_table = NULL;
>> + *pos_list = NULL;
>> /* DIRECTORY TABLE */
>> table_size = get_unaligned_le64(&sblk->fragment_table_start) -
>> get_unaligned_le64(&sblk->directory_table_start);
>> @@ -736,35 +738,31 @@ static int sqfs_read_directory_table(unsigned char **dir_table, u32 **pos_list)
>> return -ENOMEM;
>>
>> if (sqfs_disk_read(start, n_blks, dtb) < 0)
>> - goto free_dtb;
>> + goto out;
>>
>> /* Parse directory table (metadata block) header */
>> ret = sqfs_read_metablock(dtb, table_offset, &compressed, &src_len);
>> if (ret)
>> - goto free_dtb;
>> + goto out;
>>
>> /* Calculate total size to store the whole decompressed table */
>> metablks_count = sqfs_count_metablks(dtb, table_offset, table_size);
>> if (metablks_count < 1)
>> - goto free_dtb;
>> + goto out;
>>
>> *dir_table = malloc(metablks_count * SQFS_METADATA_BLOCK_SIZE);
>> if (!*dir_table)
>> - goto free_dtb;
>> + goto out;
>>
>> *pos_list = malloc(metablks_count * sizeof(u32));
>> - if (!*pos_list) {
>> - free(*dir_table);
>> - goto free_dtb;
>> - }
>> + if (!*pos_list)
>> + goto out;
>>
>> ret = sqfs_get_metablk_pos(*pos_list, dtb, table_offset,
>> metablks_count);
>> if (ret) {
>> metablks_count = -1;
>> - free(*dir_table);
>> - free(*pos_list);
>> - goto free_dtb;
>> + goto out;
>> }
>>
>> src_table = dtb + table_offset + SQFS_HEADER_SIZE;
>> @@ -780,8 +778,7 @@ static int sqfs_read_directory_table(unsigned char **dir_table, u32 **pos_list)
>> &dest_len, src_table, src_len);
>> if (ret) {
>> metablks_count = -1;
>> - free(*dir_table);
>> - goto free_dtb;
>> + goto out;
>> }
>>
>> if (dest_len < SQFS_METADATA_BLOCK_SIZE) {
>> @@ -803,7 +800,13 @@ static int sqfs_read_directory_table(unsigned char **dir_table, u32 **pos_list)
>> src_table += src_len + SQFS_HEADER_SIZE;
>> }
>>
>> -free_dtb:
>> +out:
>> + if (metablks_count < 1) {
>> + free(*dir_table);
>> + free(*pos_list);
>> + *dir_table = NULL;
>> + *pos_list = NULL;
>> + }
>> free(dtb);
>>
>> return metablks_count;
>
> Thanks,
> Miqu?l
>
Thanks !
Richard