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Re: [PATCH] block/vvfat.c fix leak when failure occurs

From: Hanna Reitz
Subject: Re: [PATCH] block/vvfat.c fix leak when failure occurs
Date: Thu, 18 Nov 2021 09:34:34 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.0

On 16.11.21 13:57, Daniella Lee wrote:
Function vvfat_open called function enable_write_target and init_directories,
and these functions malloc new memory for BDRVVVFATState::qcow_filename,
BDRVVVFATState::used_clusters, and BDRVVVFATState::cluster_buff.

When the specified folder does not exist ,it may contains memory leak.
After init_directories function is executed, the vvfat_open return -EIO,
and bdrv_open_driver goto label open_failed,
the program use g_free(bs->opaque) to release BDRVVVFATState struct
without members mentioned.

command line:
qemu-system-x86_64 -hdb <vdisk qcow file>  -usb -device usb-storage,drive=fat16
-drive file=fat:rw:fat-type=16:"<path of a host folder does not exist>",

enable_write_target called:
(gdb) bt
#0  enable_write_target (bs=0x555556f9f000, errp=0x7fffffffd780)
     at ../block/vvfat.c:3114
#1  vvfat_open (bs=0x555556f9f000, options=0x555556fa45d0,
     flags=155650, errp=0x7fffffffd780) at ../block/vvfat.c:1236
#2  bdrv_open_driver (bs=0x555556f9f000, drv=0x555556c47920 <bdrv_vvfat>,
     node_name=0x0, options=0x555556fa45d0, open_flags=155650,
     errp=0x7fffffffd890) at ../block.c:1558
#3  bdrv_open_common (bs=0x555556f9f000, file=0x0, options=0x555556fa45d0,
     errp=0x7fffffffd890) at ../block.c:1852
#4  bdrv_open_inherit (filename=0x555556f73310 "fat:rw:<dirNone>",
     reference=0x0, options=0x555556fa45d0, flags=40962, parent=0x555556f98cd0,
     child_class=0x555556b1d6a0 <child_of_bds>, child_role=19,
     errp=0x7fffffffda90) at ../block.c:3779
#5  bdrv_open_child_bs (filename=0x555556f73310 "fat:rw:<dirNone>",
     options=0x555556f9cfc0, bdref_key=0x555556239bb8 "file",
     parent=0x555556f98cd0, child_class=0x555556b1d6a0 <child_of_bds>,
     child_role=19, allow_none=true, errp=0x7fffffffda90) at ../block.c:3419
#6  bdrv_open_inherit (filename=0x555556f73310 "fat:rw:<dirNone>",
     reference=0x0, options=0x555556f9cfc0, flags=8194, parent=0x0,
     child_class=0x0, child_role=0, errp=0x555556c98c40 <error_fatal>)
     at ../block.c:3726
#7  bdrv_open (filename=0x555556f73310 "fat:rw:<dirNone>", reference=0x0,
     options=0x555556f757b0, flags=0, errp=0x555556c98c40 <error_fatal>)
     at ../block.c:3872
#8  blk_new_open (filename=0x555556f73310 "fat:rw:<dirNone>", reference=0x0,
     options=0x555556f757b0, flags=0, errp=0x555556c98c40 <error_fatal>)
     at ../block/block-backend.c:436
#9  blockdev_init (file=0x555556f73310 "fat:rw:<dirNone>",
     bs_opts=0x555556f757b0, errp=0x555556c98c40 <error_fatal>)
     at ../blockdev.c:608
#10 drive_new (all_opts=0x555556d2b700, block_default_type=IF_IDE,
     errp=0x555556c98c40 <error_fatal>) at ../blockdev.c:992

Signed-off-by: Daniella Lee <daniellalee111@gmail.com>
  block/vvfat.c | 15 +++++++++++++++
  1 file changed, 15 insertions(+)


Thanks for your patch!  Yes, that makes sense.

I believe there are some issues that should be addressed, though:

diff --git a/block/vvfat.c b/block/vvfat.c
index 05e78e3c27..454a74c5d5 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1280,7 +1280,22 @@ static int vvfat_open(BlockDriverState *bs, QDict 
*options, int flags,
ret = 0;
+    qemu_opts_del(opts);
+    return ret;

Optional: I’d drop the `ret = 0;` line and just `return 0;` here.

+    if(s->qcow_filename) {

Our coding style requires a space between `if` and the opening parenthesis.

+        g_free(s->qcow_filename);

`g_free()` checks whether the parameter given to it is `NULL`, and if so, performs a no-op.  So checking whether `s->qcow_filename != NULL` before calling `g_free()` is not necessary.

We have a script under scripts/checkpatch.pl that takes patch files as input and checks whether they conform to our coding style.  It’s really helpful, for example in these two cases it does report the issues.

+        s->qcow_filename = NULL;
+    }
+    if(s->cluster_buffer) {
+        g_free(s->cluster_buffer);
+        s->cluster_buffer = NULL;
+    }
+    if(s->used_clusters) {
+        g_free(s->used_clusters);

`s->used_clusters` is allocated with `calloc()`, so it can’t be freed with `g_free()`.  But you’re right, it should be `g_free()`-able, so the fix is to have `enable_write_target()` allocate it with `g_malloc0(size)`.

(And this made me notice that we free neither `s->used_clusters` nor `s->qcow_filename` in vvfat_close()...  Oops.)

+        s->used_clusters = NULL;
+    }
      return ret;

Finally, `enable_write_target()` frees `s->qcow_filename` on error.  That seems unnecessary now, though not wrong.  (It’s just weird that it frees that one, but not `s->used_clusters`...)


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