| Message ID | 20220819171930.16166-1-sj@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm/damon/dbgfs: avoid duplicate context directory creation | expand |
On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote: > From: Badari Pulavarty <badari.pulavarty@intel.com> > > When user tries to create a DAMON context via the DAMON debugfs > interface with a name of an already existing context, the context > directory creation silently fails but the context is added in the > internal data structure. As a result, memory could leak and DAMON > cannot be turned on. An example test case is as below: > > # cd /sys/kernel/debug/damon/ > # echo "off" > monitor_on > # echo paddr > target_ids > # echo "abc" > mk_context > # echo "abc" > mk_context > # echo $$ > abc/target_ids > # echo "on" > monitor_on <<< fails > > This commit fixes the issue by checking if the name already exist and > immediately returning '-EEXIST' in the case. > > ... > > --- a/mm/damon/dbgfs.c > +++ b/mm/damon/dbgfs.c > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx) > */ > static int dbgfs_mk_context(char *name) > { > - struct dentry *root, **new_dirs, *new_dir; > + struct dentry *root, **new_dirs, *new_dir, *dir; > struct damon_ctx **new_ctxs, *new_ctx; > > if (damon_nr_running_ctxs()) > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name) > if (!root) > return -ENOENT; > > + dir = debugfs_lookup(name, root); > + if (dir) { > + dput(dir); > + return -EEXIST; > + } > + > new_dir = debugfs_create_dir(name, root); > dbgfs_dirs[dbgfs_nr_ctxs] = new_dir; It would be simpler (and less racy) to check the debugfs_create_dir() return value for IS_ERR()?
On Fri, 19 Aug 2022 14:08:09 -0700 Andrew Morton <akpm@linux-foundation.org> wrote: > On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote: > > > From: Badari Pulavarty <badari.pulavarty@intel.com> > > > > When user tries to create a DAMON context via the DAMON debugfs > > interface with a name of an already existing context, the context > > directory creation silently fails but the context is added in the > > internal data structure. As a result, memory could leak and DAMON > > cannot be turned on. An example test case is as below: > > > > # cd /sys/kernel/debug/damon/ > > # echo "off" > monitor_on > > # echo paddr > target_ids > > # echo "abc" > mk_context > > # echo "abc" > mk_context > > # echo $$ > abc/target_ids > > # echo "on" > monitor_on <<< fails > > > > This commit fixes the issue by checking if the name already exist and > > immediately returning '-EEXIST' in the case. > > > > ... > > > > --- a/mm/damon/dbgfs.c > > +++ b/mm/damon/dbgfs.c > > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx) > > */ > > static int dbgfs_mk_context(char *name) > > { > > - struct dentry *root, **new_dirs, *new_dir; > > + struct dentry *root, **new_dirs, *new_dir, *dir; > > struct damon_ctx **new_ctxs, *new_ctx; > > > > if (damon_nr_running_ctxs()) > > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name) > > if (!root) > > return -ENOENT; > > > > + dir = debugfs_lookup(name, root); > > + if (dir) { > > + dput(dir); > > + return -EEXIST; > > + } > > + > > new_dir = debugfs_create_dir(name, root); > > dbgfs_dirs[dbgfs_nr_ctxs] = new_dir; > > It would be simpler (and less racy) to check the debugfs_create_dir() > return value for IS_ERR()? I was merely following Greg's previous advice for ignoring the return value[1] of the function, but I might misunderstanding his intention, so CC-ing Greg. Greg, may I ask your opinion? [1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/ Thanks, SJ
On Fri, 19 Aug 2022 21:16:31 +0000 SeongJae Park <sj@kernel.org> wrote: > > It would be simpler (and less racy) to check the debugfs_create_dir() > > return value for IS_ERR()? > > I was merely following Greg's previous advice for ignoring the return value[1] > of the function, but I might misunderstanding his intention, so CC-ing Greg. > Greg, may I ask your opinion? > > [1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/ Thing is, the correct functioning of the debugfs interfaces is utterly critical to damon. And that's apart from these memory leak and oops-we-killed-damon issues. So damon simply cannot ignore the state of its debugfs interfaces and keep going along - because if something goes wrong at the debugfs layer, damon is dead and useless and the machine needs a reboot. Perhaps this means that damon should not be using debugfs for its interfaces at all. Or it means that the debugfs interfaces are misdesigned. I go with the latter, which, alas, also affirms the former. From a quick scan it appears that a significant minority (20%?) of drivers are checking the debugfs_create_dir() return value.
On Fri, 19 Aug 2022 15:44:51 -0700 Andrew Morton <akpm@linux-foundation.org> wrote: > On Fri, 19 Aug 2022 21:16:31 +0000 SeongJae Park <sj@kernel.org> wrote: > > > > It would be simpler (and less racy) to check the debugfs_create_dir() > > > return value for IS_ERR()? > > > > I was merely following Greg's previous advice for ignoring the return value[1] > > of the function, but I might misunderstanding his intention, so CC-ing Greg. > > Greg, may I ask your opinion? > > > > [1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/ > > Thing is, the correct functioning of the debugfs interfaces is utterly > critical to damon. And that's apart from these memory leak and > oops-we-killed-damon issues. > > So damon simply cannot ignore the state of its debugfs interfaces and > keep going along - because if something goes wrong at the debugfs > layer, damon is dead and useless and the machine needs a reboot. > > Perhaps this means that damon should not be using debugfs for its > interfaces at all. Or it means that the debugfs interfaces are > misdesigned. I go with the latter, which, alas, also affirms the > former. I'd save my word about the latter, but agreed on the former. Fortunately we already have an alternative (DAMON sysfs interface), and the debugfs interface deprecation plan was announced for a while ago. Not sure if the deprecation will be well as hoped, though. > > From a quick scan it appears that a significant minority (20%?) of > drivers are checking the debugfs_create_dir() return value. Maybe partly owing to Greg's previous efforts for removing the checks[1,2]? Anyway, based on the previous discussions, I'd expect Greg might prefer not checking the return code. Anyway, waiting for his opinion. [1] https://lore.kernel.org/all/20190122152151.16139-14-gregkh@linuxfoundation.org/ [2] https://lore.kernel.org/lkml/20190122152151.16139-7-gregkh@linuxfoundation.org/ Thanks, SJ
On 8/20/22 00:19, SeongJae Park wrote: > From: Badari Pulavarty <badari.pulavarty@intel.com> > > When user tries to create a DAMON context via the DAMON debugfs > interface with a name of an already existing context, the context > directory creation silently fails but the context is added in the > internal data structure. As a result, memory could leak and DAMON > cannot be turned on. An example test case is as below: > > # cd /sys/kernel/debug/damon/ > # echo "off" > monitor_on > # echo paddr > target_ids > # echo "abc" > mk_context > # echo "abc" > mk_context > # echo $$ > abc/target_ids > # echo "on" > monitor_on <<< fails > > This commit fixes the issue by checking if the name already exist and > immediately returning '-EEXIST' in the case. > Meh... SJ, I have seen most (if not all of) your patches uses descriptive mood instead of imperative. Better say "Fix the issue by checking ...".
On Fri, Aug 19, 2022 at 02:08:09PM -0700, Andrew Morton wrote: > On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote: > > > From: Badari Pulavarty <badari.pulavarty@intel.com> > > > > When user tries to create a DAMON context via the DAMON debugfs > > interface with a name of an already existing context, the context > > directory creation silently fails but the context is added in the > > internal data structure. As a result, memory could leak and DAMON > > cannot be turned on. An example test case is as below: > > > > # cd /sys/kernel/debug/damon/ > > # echo "off" > monitor_on > > # echo paddr > target_ids > > # echo "abc" > mk_context > > # echo "abc" > mk_context > > # echo $$ > abc/target_ids > > # echo "on" > monitor_on <<< fails > > > > This commit fixes the issue by checking if the name already exist and > > immediately returning '-EEXIST' in the case. > > > > ... > > > > --- a/mm/damon/dbgfs.c > > +++ b/mm/damon/dbgfs.c > > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx) > > */ > > static int dbgfs_mk_context(char *name) > > { > > - struct dentry *root, **new_dirs, *new_dir; > > + struct dentry *root, **new_dirs, *new_dir, *dir; > > struct damon_ctx **new_ctxs, *new_ctx; > > > > if (damon_nr_running_ctxs()) > > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name) > > if (!root) > > return -ENOENT; > > > > + dir = debugfs_lookup(name, root); > > + if (dir) { > > + dput(dir); > > + return -EEXIST; > > + } > > + > > new_dir = debugfs_create_dir(name, root); > > dbgfs_dirs[dbgfs_nr_ctxs] = new_dir; > > It would be simpler (and less racy) to check the debugfs_create_dir() > return value for IS_ERR()? > Yes, if you _HAVE_ to know if the code works properly (i.e. because your feature totally depends on debugfs like damon does), or you have a potential duplicate name like this, then sure, check the return value and do something based on it. It's odd enough that you should put a comment above the check just so I don't go and send a patch to delete it later on :) thanks, greg k-h
Hi Greg, On Sat, 20 Aug 2022 19:32:37 +0200 Greg KH <gregkh@linuxfoundation.org> wrote: > On Fri, Aug 19, 2022 at 02:08:09PM -0700, Andrew Morton wrote: > > On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote: > > > > > From: Badari Pulavarty <badari.pulavarty@intel.com> > > > > > > When user tries to create a DAMON context via the DAMON debugfs > > > interface with a name of an already existing context, the context > > > directory creation silently fails but the context is added in the > > > internal data structure. As a result, memory could leak and DAMON > > > cannot be turned on. An example test case is as below: > > > > > > # cd /sys/kernel/debug/damon/ > > > # echo "off" > monitor_on > > > # echo paddr > target_ids > > > # echo "abc" > mk_context > > > # echo "abc" > mk_context > > > # echo $$ > abc/target_ids > > > # echo "on" > monitor_on <<< fails > > > > > > This commit fixes the issue by checking if the name already exist and > > > immediately returning '-EEXIST' in the case. > > > > > > ... > > > > > > --- a/mm/damon/dbgfs.c > > > +++ b/mm/damon/dbgfs.c > > > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx) > > > */ > > > static int dbgfs_mk_context(char *name) > > > { > > > - struct dentry *root, **new_dirs, *new_dir; > > > + struct dentry *root, **new_dirs, *new_dir, *dir; > > > struct damon_ctx **new_ctxs, *new_ctx; > > > > > > if (damon_nr_running_ctxs()) > > > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name) > > > if (!root) > > > return -ENOENT; > > > > > > + dir = debugfs_lookup(name, root); > > > + if (dir) { > > > + dput(dir); > > > + return -EEXIST; > > > + } > > > + > > > new_dir = debugfs_create_dir(name, root); > > > dbgfs_dirs[dbgfs_nr_ctxs] = new_dir; > > > > It would be simpler (and less racy) to check the debugfs_create_dir() > > return value for IS_ERR()? > > > > Yes, if you _HAVE_ to know if the code works properly (i.e. because your > feature totally depends on debugfs like damon does), or you have a > potential duplicate name like this, then sure, check the return value > and do something based on it. > > It's odd enough that you should put a comment above the check just so I > don't go and send a patch to delete it later on :) Thank you for the kind explanation, Greg. I will revise this patch to simply check the return value with a comment noticing it's really needed due to the potential duplicate names. Thanks, SJ > > thanks, > > greg k-h >
diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c index 51d67c8050dd..364b44063c2f 100644 --- a/mm/damon/dbgfs.c +++ b/mm/damon/dbgfs.c @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx) */ static int dbgfs_mk_context(char *name) { - struct dentry *root, **new_dirs, *new_dir; + struct dentry *root, **new_dirs, *new_dir, *dir; struct damon_ctx **new_ctxs, *new_ctx; if (damon_nr_running_ctxs()) @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name) if (!root) return -ENOENT; + dir = debugfs_lookup(name, root); + if (dir) { + dput(dir); + return -EEXIST; + } + new_dir = debugfs_create_dir(name, root); dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;