| Message ID | 20230125233554.153109-2-surenb@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | introduce vm_flags modifier functions | expand |
On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > errors when we add a const modifier to vma->vm_flags. > > ... > > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > * orig->shared.rb may be modified concurrently, but the clone > * will be reinitialized. > */ > - *new = data_race(*orig); > + memcpy(new, orig, sizeof(*new)); The data_race() removal is unchangelogged? > INIT_LIST_HEAD(&new->anon_vma_chain); > dup_anon_vma_name(orig, new); > }
On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > errors when we add a const modifier to vma->vm_flags. > > > > ... > > > > --- a/kernel/fork.c > > +++ b/kernel/fork.c > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > * orig->shared.rb may be modified concurrently, but the clone > > * will be reinitialized. > > */ > > - *new = data_race(*orig); > > + memcpy(new, orig, sizeof(*new)); > > The data_race() removal is unchangelogged? True. I'll add a note in the changelog about that. Ideally I would like to preserve it but I could not find a way to do that. > > > INIT_LIST_HEAD(&new->anon_vma_chain); > > dup_anon_vma_name(orig, new); > > } >
On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > errors when we add a const modifier to vma->vm_flags. > > > > > > ... > > > > > > --- a/kernel/fork.c > > > +++ b/kernel/fork.c > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > * orig->shared.rb may be modified concurrently, but the clone > > > * will be reinitialized. > > > */ > > > - *new = data_race(*orig); > > > + memcpy(new, orig, sizeof(*new)); > > > > The data_race() removal is unchangelogged? > > True. I'll add a note in the changelog about that. Ideally I would > like to preserve it but I could not find a way to do that. > Perhaps Paul can comment? I wonder if KCSAN knows how to detect this race, given that it's now in a memcpy. I assume so.
On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > ... > > > > > > > > --- a/kernel/fork.c > > > > +++ b/kernel/fork.c > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > * will be reinitialized. > > > > */ > > > > - *new = data_race(*orig); > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > The data_race() removal is unchangelogged? > > > > True. I'll add a note in the changelog about that. Ideally I would > > like to preserve it but I could not find a way to do that. > > > > Perhaps Paul can comment? > > I wonder if KCSAN knows how to detect this race, given that it's now in > a memcpy. I assume so. data_race() is just wrapping an expression around __kcsan_[en|dis]able_current and ensuring the expression is evaluated once and returning the correct type. I believe the following should be sufficient. diff --git a/kernel/fork.c b/kernel/fork.c index 9f7fe3541897..1b30ee568e02 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -472,7 +472,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) * orig->shared.rb may be modified concurrently, but the clone * will be reinitialized. */ - *new = data_race(*orig); + data_race(memcpy(new, orig, sizeof(*new))); INIT_LIST_HEAD(&new->anon_vma_chain); dup_anon_vma_name(orig, new); } I don't see how memcpy could automagically figure out whether the memcpy is prone to races or not in an arbitrary context. Assuming using data_race this way is ok then Acked-by: Mel Gorman <mgorman@techsingularity.net>
On Thu, Jan 26, 2023 at 3:52 AM Mel Gorman <mgorman@techsingularity.net> wrote: > > On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > > > ... > > > > > > > > > > --- a/kernel/fork.c > > > > > +++ b/kernel/fork.c > > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > > * will be reinitialized. > > > > > */ > > > > > - *new = data_race(*orig); > > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > > > The data_race() removal is unchangelogged? > > > > > > True. I'll add a note in the changelog about that. Ideally I would > > > like to preserve it but I could not find a way to do that. > > > > > > > Perhaps Paul can comment? > > > > I wonder if KCSAN knows how to detect this race, given that it's now in > > a memcpy. I assume so. > > data_race() is just wrapping an expression around > __kcsan_[en|dis]able_current and ensuring the expression is evaluated once > and returning the correct type. I believe the following should be sufficient. Thanks for the suggestion, Mel! I'll try that. > > diff --git a/kernel/fork.c b/kernel/fork.c > index 9f7fe3541897..1b30ee568e02 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -472,7 +472,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > * orig->shared.rb may be modified concurrently, but the clone > * will be reinitialized. > */ > - *new = data_race(*orig); > + data_race(memcpy(new, orig, sizeof(*new))); > INIT_LIST_HEAD(&new->anon_vma_chain); > dup_anon_vma_name(orig, new); > } > > I don't see how memcpy could automagically figure out whether the memcpy > is prone to races or not in an arbitrary context. > > Assuming using data_race this way is ok then > > Acked-by: Mel Gorman <mgorman@techsingularity.net> Thanks! > > -- > Mel Gorman > SUSE Labs
On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > ... > > > > > > > > --- a/kernel/fork.c > > > > +++ b/kernel/fork.c > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > * will be reinitialized. > > > > */ > > > > - *new = data_race(*orig); > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > The data_race() removal is unchangelogged? > > > > True. I'll add a note in the changelog about that. Ideally I would > > like to preserve it but I could not find a way to do that. > > Perhaps Paul can comment? > > I wonder if KCSAN knows how to detect this race, given that it's now in > a memcpy. I assume so. I ran an experiment memcpy()ing between a static array and an onstack array, and KCSAN did not complain. But maybe I was setting it up wrong. This is what I did: long myid = (long)arg; /* different value for each task */ static unsigned long z1[10] = { 0 }; unsigned long z2[10]; ... memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0])); for (zi = 0; zi < ARRAY_SIZE(z1); zi++) z2[zi] += myid; memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0])); Adding Marco on CC for his thoughts. Thanx, Paul
On Thu, Jan 26, 2023 at 09:27AM -0800, Paul E. McKenney wrote: > On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > > > ... > > > > > > > > > > --- a/kernel/fork.c > > > > > +++ b/kernel/fork.c > > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > > * will be reinitialized. > > > > > */ > > > > > - *new = data_race(*orig); > > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > > > The data_race() removal is unchangelogged? > > > > > > True. I'll add a note in the changelog about that. Ideally I would > > > like to preserve it but I could not find a way to do that. > > > > Perhaps Paul can comment? > > > > I wonder if KCSAN knows how to detect this race, given that it's now in > > a memcpy. I assume so. > > I ran an experiment memcpy()ing between a static array and an onstack > array, and KCSAN did not complain. But maybe I was setting it up wrong. > > This is what I did: > > long myid = (long)arg; /* different value for each task */ > static unsigned long z1[10] = { 0 }; > unsigned long z2[10]; > > ... > > memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0])); > for (zi = 0; zi < ARRAY_SIZE(z1); zi++) > z2[zi] += myid; > memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0])); > > Adding Marco on CC for his thoughts. ( Sorry for not seeing it earlier - just saw this by chance. ) memcpy() data races will be detected as of (given a relatively recent Clang compiler): https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c201739beef Also beware that the compiler is free to "optimize" things by either inlining memcpy() (turning an explicit memcpy() into just a bunch of loads/stores), or outline plain assignments into memcpy() calls. So the only way to be sure what ends up there is to look at the disassembled code. The data_race() was introduced by: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cda099b37d716 It says: "vm_area_dup() blindly copies all fields of original VMA to the new one. This includes coping vm_area_struct::shared.rb which is normally protected by i_mmap_lock. But this is fine because the read value will be overwritten on the following __vma_link_file() under proper protection. Thus, mark it as an intentional data race and insert a few assertions for the fields that should not be modified concurrently." And as far as I can tell this hasn't changed. Thanks, -- Marco
On Tue, Feb 7, 2023 at 9:16 AM Marco Elver <elver@google.com> wrote: > > On Thu, Jan 26, 2023 at 09:27AM -0800, Paul E. McKenney wrote: > > On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > > > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > > > > > ... > > > > > > > > > > > > --- a/kernel/fork.c > > > > > > +++ b/kernel/fork.c > > > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > > > * will be reinitialized. > > > > > > */ > > > > > > - *new = data_race(*orig); > > > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > > > > > The data_race() removal is unchangelogged? > > > > > > > > True. I'll add a note in the changelog about that. Ideally I would > > > > like to preserve it but I could not find a way to do that. > > > > > > Perhaps Paul can comment? > > > > > > I wonder if KCSAN knows how to detect this race, given that it's now in > > > a memcpy. I assume so. > > > > I ran an experiment memcpy()ing between a static array and an onstack > > array, and KCSAN did not complain. But maybe I was setting it up wrong. > > > > This is what I did: > > > > long myid = (long)arg; /* different value for each task */ > > static unsigned long z1[10] = { 0 }; > > unsigned long z2[10]; > > > > ... > > > > memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0])); > > for (zi = 0; zi < ARRAY_SIZE(z1); zi++) > > z2[zi] += myid; > > memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0])); > > > > Adding Marco on CC for his thoughts. > > ( Sorry for not seeing it earlier - just saw this by chance. ) > > memcpy() data races will be detected as of (given a relatively recent > Clang compiler): > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c201739beef > > Also beware that the compiler is free to "optimize" things by either > inlining memcpy() (turning an explicit memcpy() into just a bunch of > loads/stores), or outline plain assignments into memcpy() calls. So the > only way to be sure what ends up there is to look at the disassembled > code. > > The data_race() was introduced by: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cda099b37d716 > > It says: > "vm_area_dup() blindly copies all fields of original VMA to the new one. > This includes coping vm_area_struct::shared.rb which is normally > protected by i_mmap_lock. But this is fine because the read value will > be overwritten on the following __vma_link_file() under proper > protection. Thus, mark it as an intentional data race and insert a few > assertions for the fields that should not be modified concurrently." > > And as far as I can tell this hasn't changed. Thanks for the feedback, Marco! So, IIUC Mel's proposal to use data_race(memcpy(new, orig, sizeof(*new))); is fine in this case, right? Thanks, Suren. > > Thanks, > -- Marco
On Tue, 7 Feb 2023 at 18:24, Suren Baghdasaryan <surenb@google.com> wrote: > > On Tue, Feb 7, 2023 at 9:16 AM Marco Elver <elver@google.com> wrote: > > > > On Thu, Jan 26, 2023 at 09:27AM -0800, Paul E. McKenney wrote: > > > On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > > > > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote: > > > > > > > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@google.com> wrote: > > > > > > > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > > > > > > > ... > > > > > > > > > > > > > > --- a/kernel/fork.c > > > > > > > +++ b/kernel/fork.c > > > > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > > > > * will be reinitialized. > > > > > > > */ > > > > > > > - *new = data_race(*orig); > > > > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > > > > > > > The data_race() removal is unchangelogged? > > > > > > > > > > True. I'll add a note in the changelog about that. Ideally I would > > > > > like to preserve it but I could not find a way to do that. > > > > > > > > Perhaps Paul can comment? > > > > > > > > I wonder if KCSAN knows how to detect this race, given that it's now in > > > > a memcpy. I assume so. > > > > > > I ran an experiment memcpy()ing between a static array and an onstack > > > array, and KCSAN did not complain. But maybe I was setting it up wrong. > > > > > > This is what I did: > > > > > > long myid = (long)arg; /* different value for each task */ > > > static unsigned long z1[10] = { 0 }; > > > unsigned long z2[10]; > > > > > > ... > > > > > > memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0])); > > > for (zi = 0; zi < ARRAY_SIZE(z1); zi++) > > > z2[zi] += myid; > > > memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0])); > > > > > > Adding Marco on CC for his thoughts. > > > > ( Sorry for not seeing it earlier - just saw this by chance. ) > > > > memcpy() data races will be detected as of (given a relatively recent > > Clang compiler): > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c201739beef > > > > Also beware that the compiler is free to "optimize" things by either > > inlining memcpy() (turning an explicit memcpy() into just a bunch of > > loads/stores), or outline plain assignments into memcpy() calls. So the > > only way to be sure what ends up there is to look at the disassembled > > code. > > > > The data_race() was introduced by: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cda099b37d716 > > > > It says: > > "vm_area_dup() blindly copies all fields of original VMA to the new one. > > This includes coping vm_area_struct::shared.rb which is normally > > protected by i_mmap_lock. But this is fine because the read value will > > be overwritten on the following __vma_link_file() under proper > > protection. Thus, mark it as an intentional data race and insert a few > > assertions for the fields that should not be modified concurrently." > > > > And as far as I can tell this hasn't changed. > > Thanks for the feedback, Marco! > So, IIUC Mel's proposal to use data_race(memcpy(new, orig, > sizeof(*new))); is fine in this case, right? Yes, that'd work.
diff --git a/kernel/fork.c b/kernel/fork.c index 6683c1b0f460..a531901859d9 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) * orig->shared.rb may be modified concurrently, but the clone * will be reinitialized. */ - *new = data_race(*orig); + memcpy(new, orig, sizeof(*new)); INIT_LIST_HEAD(&new->anon_vma_chain); dup_anon_vma_name(orig, new); }
Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler errors when we add a const modifier to vma->vm_flags. Signed-off-by: Suren Baghdasaryan <surenb@google.com> --- kernel/fork.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)