| Message ID | 20190927153949.29870-1-msys.mizuma@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] arm64/sve: Fix wrong free for task->thread.sve_state | expand |
On Fri, Sep 27, 2019 at 11:39:49AM -0400, Masayoshi Mizuma wrote: > From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > The system which has SVE feature crashed because of > the memory pointed by task->thread.sve_state was destroyed > by someone. > > That is because sve_state is freed while the forking the > child process. The child process has the pointer of sve_state > which is same as the parent's because the child's task_struct > is copied from the parent's one. If the copy_process() > fails as an error on somewhere, for example, copy_creds(), > then the sve_state is freed even if the parent is alive. > The flow is as follows. > > copy_process > p = dup_task_struct > => arch_dup_task_struct > *dst = *src; // copy the entire region. > : > retval = copy_creds > if (retval < 0) > goto bad_fork_free; > : > bad_fork_free: > ... > delayed_free_task(p); > => free_task > => arch_release_task_struct > => fpsimd_release_task > => __sve_free > => kfree(task->thread.sve_state); > // free the parent's sve_state > > Move child's sve_state = NULL and clearing TIF_SVE flag > to arch_dup_task_struct() so that the child doesn't free the > parent's one. You could also add: --8<-- There is no need to wait until copy_process() to clear TIF_SVE for dst, becuase the thread flags for dst are initialized already by copying the src task_struct. This change simplifies the code, so get rid of comments that are no longer needed. -->8-- > > Cc: stable@vger.kernel.org Since SVE only exists from v4.15, it may be helpful to specify that, i.e., replace that Cc line with: Cc: <stable@vger.kernel.org> # 4.15.x- Otherwise, I'm happy to see this applied, but I'd like somebody to confirm that this change definitely fixes the bug. Cheers ---Dave [...] > Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") > Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> > Suggested-by: Dave Martin <Dave.Martin@arm.com> > --- > arch/arm64/kernel/process.c | 21 ++++----------------- > 1 file changed, 4 insertions(+), 17 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index f674f28df..6937f5935 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) > fpsimd_release_task(tsk); > } > > -/* > - * src and dst may temporarily have aliased sve_state after task_struct > - * is copied. We cannot fix this properly here, because src may have > - * live SVE state and dst's thread_info may not exist yet, so tweaking > - * either src's or dst's TIF_SVE is not safe. > - * > - * The unaliasing is done in copy_thread() instead. This works because > - * dst is not schedulable or traceable until both of these functions > - * have been called. > - */ > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) > { > if (current->mm) > fpsimd_preserve_current_state(); > *dst = *src; > > + BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); > + dst->thread.sve_state = NULL; > + clear_tsk_thread_flag(dst, TIF_SVE); > + > return 0; > } > > @@ -351,13 +345,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, > > memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context)); > > - /* > - * Unalias p->thread.sve_state (if any) from the parent task > - * and disable discard SVE state for p: > - */ > - clear_tsk_thread_flag(p, TIF_SVE); > - p->thread.sve_state = NULL; > - > /* > * In case p was allocated the same task_struct pointer as some > * other recently-exited task, make sure p is disassociated from > -- > 2.18.1 > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
Hi Dave, On 27/09/2019 17:15, Dave Martin wrote: > On Fri, Sep 27, 2019 at 11:39:49AM -0400, Masayoshi Mizuma wrote: >> From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> >> >> The system which has SVE feature crashed because of >> the memory pointed by task->thread.sve_state was destroyed >> by someone. >> >> That is because sve_state is freed while the forking the >> child process. The child process has the pointer of sve_state >> which is same as the parent's because the child's task_struct >> is copied from the parent's one. If the copy_process() >> fails as an error on somewhere, for example, copy_creds(), >> then the sve_state is freed even if the parent is alive. >> The flow is as follows. >> >> copy_process >> p = dup_task_struct >> => arch_dup_task_struct >> *dst = *src; // copy the entire region. >> : >> retval = copy_creds >> if (retval < 0) >> goto bad_fork_free; >> : >> bad_fork_free: >> ... >> delayed_free_task(p); >> => free_task >> => arch_release_task_struct >> => fpsimd_release_task >> => __sve_free >> => kfree(task->thread.sve_state); >> // free the parent's sve_state >> >> Move child's sve_state = NULL and clearing TIF_SVE flag >> to arch_dup_task_struct() so that the child doesn't free the >> parent's one. > > You could also add: > > --8<-- > There is no need to wait until copy_process() to clear TIF_SVE for > dst, becuase the thread flags for dst are initialized already by > copying the src task_struct. > > This change simplifies the code, so get rid of comments that are no > longer needed. > -->8-- > >> >> Cc: stable@vger.kernel.org > > Since SVE only exists from v4.15, it may be helpful to specify that, > i.e., replace that Cc line with: > > Cc: <stable@vger.kernel.org> # 4.15.x- > > > Otherwise, I'm happy to see this applied, but I'd like somebody to > confirm that this change definitely fixes the bug. I am working on a reproducer for this. So I should be able to test it. Cheers,
Hi, On 27/09/2019 16:39, Masayoshi Mizuma wrote: > From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > The system which has SVE feature crashed because of > the memory pointed by task->thread.sve_state was destroyed > by someone. > > That is because sve_state is freed while the forking the > child process. The child process has the pointer of sve_state > which is same as the parent's because the child's task_struct > is copied from the parent's one. If the copy_process() > fails as an error on somewhere, for example, copy_creds(), > then the sve_state is freed even if the parent is alive. > The flow is as follows. > > copy_process > p = dup_task_struct > => arch_dup_task_struct > *dst = *src; // copy the entire region. > : > retval = copy_creds > if (retval < 0) > goto bad_fork_free; > : > bad_fork_free: > ... > delayed_free_task(p); > => free_task > => arch_release_task_struct > => fpsimd_release_task > => __sve_free > => kfree(task->thread.sve_state); > // free the parent's sve_state > > Move child's sve_state = NULL and clearing TIF_SVE flag > to arch_dup_task_struct() so that the child doesn't free the > parent's one. > > Cc: stable@vger.kernel.org > Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") Looking at the log, it looks like THREAD_INFO_IN_TASK was selected before the bc0ee4760364. So it should be fine to backport for all the Linux tree contain this commit. > Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> > Suggested-by: Dave Martin <Dave.Martin@arm.com> I have tested the patch and can confirm that double-free disappeared after the patch is applied: Tested-by: Julien Grall <julien.grall@arm.com> See below for a few comments. > --- > arch/arm64/kernel/process.c | 21 ++++----------------- > 1 file changed, 4 insertions(+), 17 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index f674f28df..6937f5935 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) > fpsimd_release_task(tsk); > } > > -/* > - * src and dst may temporarily have aliased sve_state after task_struct > - * is copied. We cannot fix this properly here, because src may have > - * live SVE state and dst's thread_info may not exist yet, so tweaking > - * either src's or dst's TIF_SVE is not safe. > - * > - * The unaliasing is done in copy_thread() instead. This works because > - * dst is not schedulable or traceable until both of these functions > - * have been called. > - */ It would be good to explain in the commit message why tweaking "dst" in arch_dup_task_struct() is fine. From my understanding, Arm64 used to have thread_info on the stack. So it would not be possible to clear TIF_SVE until the stack is initialized. Now that the thread_info is part of the task, it should be valid to modify the flag from arch_dup_task_struct(). Note that technically, TIF_SVE does not need to be cleared from arch_dup_task_struct(). It could also be done from copy_thread(). But it is easier to keep the both changes together. > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) > { > if (current->mm) > fpsimd_preserve_current_state(); > *dst = *src; > > + BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); You may want to add a comment on top explaining why TIF_SVE is cleared here. > + dst->thread.sve_state = NULL; > + clear_tsk_thread_flag(dst, TIF_SVE); > + > return 0; > } > > @@ -351,13 +345,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, > > memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context)); > > - /* > - * Unalias p->thread.sve_state (if any) from the parent task > - * and disable discard SVE state for p: > - */ > - clear_tsk_thread_flag(p, TIF_SVE); > - p->thread.sve_state = NULL; > - > /* > * In case p was allocated the same task_struct pointer as some > * other recently-exited task, make sure p is disassociated from > Cheers,
On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote: > Hi, > > On 27/09/2019 16:39, Masayoshi Mizuma wrote: > >From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > > >The system which has SVE feature crashed because of > >the memory pointed by task->thread.sve_state was destroyed > >by someone. > > > >That is because sve_state is freed while the forking the > >child process. The child process has the pointer of sve_state > >which is same as the parent's because the child's task_struct > >is copied from the parent's one. If the copy_process() > >fails as an error on somewhere, for example, copy_creds(), > >then the sve_state is freed even if the parent is alive. > >The flow is as follows. > > > >copy_process > > p = dup_task_struct > > => arch_dup_task_struct > > *dst = *src; // copy the entire region. > >: > > retval = copy_creds > > if (retval < 0) > > goto bad_fork_free; > >: > >bad_fork_free: > >... > > delayed_free_task(p); > > => free_task > > => arch_release_task_struct > > => fpsimd_release_task > > => __sve_free > > => kfree(task->thread.sve_state); > > // free the parent's sve_state > > > >Move child's sve_state = NULL and clearing TIF_SVE flag > >to arch_dup_task_struct() so that the child doesn't free the > >parent's one. > > > >Cc: stable@vger.kernel.org > >Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") > > Looking at the log, it looks like THREAD_INFO_IN_TASK was selected before > the bc0ee4760364. So it should be fine to backport for all the Linux tree > contain this commit. > > >Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > >Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> > >Suggested-by: Dave Martin <Dave.Martin@arm.com> > > I have tested the patch and can confirm that double-free disappeared after > the patch is applied: > > Tested-by: Julien Grall <julien.grall@arm.com> Good to have that confirmed -- thanks for verifying. [...] > >--- > > arch/arm64/kernel/process.c | 21 ++++----------------- > > 1 file changed, 4 insertions(+), 17 deletions(-) > > > >diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > >index f674f28df..6937f5935 100644 > >--- a/arch/arm64/kernel/process.c > >+++ b/arch/arm64/kernel/process.c > >@@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) > > fpsimd_release_task(tsk); > > } > >-/* > >- * src and dst may temporarily have aliased sve_state after task_struct > >- * is copied. We cannot fix this properly here, because src may have > >- * live SVE state and dst's thread_info may not exist yet, so tweaking > >- * either src's or dst's TIF_SVE is not safe. > >- * > >- * The unaliasing is done in copy_thread() instead. This works because > >- * dst is not schedulable or traceable until both of these functions > >- * have been called. > >- */ > > It would be good to explain in the commit message why tweaking "dst" in > arch_dup_task_struct() is fine. > > From my understanding, Arm64 used to have thread_info on the stack. So it > would not be possible to clear TIF_SVE until the stack is initialized. > > Now that the thread_info is part of the task, it should be valid to modify > the flag from arch_dup_task_struct(). > > Note that technically, TIF_SVE does not need to be cleared from > arch_dup_task_struct(). It could also be done from copy_thread(). But it is > easier to keep the both changes together. > > > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) > > { > > if (current->mm) > > fpsimd_preserve_current_state(); > > *dst = *src; Ack, some more explanation would be a good idea here. Maybe the following comments are sufficient? /* We rely on the above assingment to initialise dst's thread_flags: */ > >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); > and /* * Detach src's sve_state (if any) from dst so that it does not * get erroneously used or freed prematurely. dst's sve_state * will be allocated on demand later on if dst uses SVE. * For consistency, also clear TIF_SVE here: this could be done * later in copy_process(), but to avoid tripping up future * maintainers it is best not to leave TIF_SVE and sve_state in * an inconsistent state, even temporarily. */ > >+ dst->thread.sve_state = NULL; > >+ clear_tsk_thread_flag(dst, TIF_SVE); (TIF_SVE should not usually be set in the first place of course, since we are in a fork() or clone() syscall in src. This may not be true if a task is created using kernel_thread() while running in the context of some user task that entered the kernel due to a trap or syscall -- though probably nobody should be doing that.) [...] Cheers ---DavE
Hi Julien and Dave, On Mon, Sep 30, 2019 at 02:02:46PM +0100, Dave Martin wrote: > On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote: > > Hi, > > > > On 27/09/2019 16:39, Masayoshi Mizuma wrote: > > >From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > > > > >The system which has SVE feature crashed because of > > >the memory pointed by task->thread.sve_state was destroyed > > >by someone. > > > > > >That is because sve_state is freed while the forking the > > >child process. The child process has the pointer of sve_state > > >which is same as the parent's because the child's task_struct > > >is copied from the parent's one. If the copy_process() > > >fails as an error on somewhere, for example, copy_creds(), > > >then the sve_state is freed even if the parent is alive. > > >The flow is as follows. > > > > > >copy_process > > > p = dup_task_struct > > > => arch_dup_task_struct > > > *dst = *src; // copy the entire region. > > >: > > > retval = copy_creds > > > if (retval < 0) > > > goto bad_fork_free; > > >: > > >bad_fork_free: > > >... > > > delayed_free_task(p); > > > => free_task > > > => arch_release_task_struct > > > => fpsimd_release_task > > > => __sve_free > > > => kfree(task->thread.sve_state); > > > // free the parent's sve_state > > > > > >Move child's sve_state = NULL and clearing TIF_SVE flag > > >to arch_dup_task_struct() so that the child doesn't free the > > >parent's one. > > > > > >Cc: stable@vger.kernel.org > > >Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") > > > > Looking at the log, it looks like THREAD_INFO_IN_TASK was selected before > > the bc0ee4760364. So it should be fine to backport for all the Linux tree > > contain this commit. I think this patch is needed for the kernel has SVE support. I'll add the Cc tag as Dave said: Cc: stable@vger.kernel.org # 4.15+ So, I suppose this patch will be backported to stables 5.3.X, 5.2.X and longterm 4.19.X. Does this make sense? > > > > >Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > >Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> > > >Suggested-by: Dave Martin <Dave.Martin@arm.com> > > > > I have tested the patch and can confirm that double-free disappeared after > > the patch is applied: > > > > Tested-by: Julien Grall <julien.grall@arm.com> Thank you so much! > > Good to have that confirmed -- thanks for verifying. > > [...] > > > >--- > > > arch/arm64/kernel/process.c | 21 ++++----------------- > > > 1 file changed, 4 insertions(+), 17 deletions(-) > > > > > >diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > > >index f674f28df..6937f5935 100644 > > >--- a/arch/arm64/kernel/process.c > > >+++ b/arch/arm64/kernel/process.c > > >@@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) > > > fpsimd_release_task(tsk); > > > } > > >-/* > > >- * src and dst may temporarily have aliased sve_state after task_struct > > >- * is copied. We cannot fix this properly here, because src may have > > >- * live SVE state and dst's thread_info may not exist yet, so tweaking > > >- * either src's or dst's TIF_SVE is not safe. > > >- * > > >- * The unaliasing is done in copy_thread() instead. This works because > > >- * dst is not schedulable or traceable until both of these functions > > >- * have been called. > > >- */ > > > > It would be good to explain in the commit message why tweaking "dst" in > > arch_dup_task_struct() is fine. > > > > From my understanding, Arm64 used to have thread_info on the stack. So it > > would not be possible to clear TIF_SVE until the stack is initialized. > > > > Now that the thread_info is part of the task, it should be valid to modify > > the flag from arch_dup_task_struct(). > > > > Note that technically, TIF_SVE does not need to be cleared from > > arch_dup_task_struct(). It could also be done from copy_thread(). But it is > > easier to keep the both changes together. Thanks, let me add some comments to the commit log. > > > > > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) > > > { > > > if (current->mm) > > > fpsimd_preserve_current_state(); > > > *dst = *src; > > Ack, some more explanation would be a good idea here. > > Maybe the following comments are sufficient? > > /* We rely on the above assingment to initialise dst's thread_flags: */ Thanks, I'll add this comment. > > > >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); > > > > and > > /* > * Detach src's sve_state (if any) from dst so that it does not > * get erroneously used or freed prematurely. dst's sve_state > * will be allocated on demand later on if dst uses SVE. > * For consistency, also clear TIF_SVE here: this could be done > * later in copy_process(), but to avoid tripping up future > * maintainers it is best not to leave TIF_SVE and sve_state in > * an inconsistent state, even temporarily. > */ I'll add this comments. > > > >+ dst->thread.sve_state = NULL; > > >+ clear_tsk_thread_flag(dst, TIF_SVE); > > (TIF_SVE should not usually be set in the first place of course, since > we are in a fork() or clone() syscall in src. This may not be true if > a task is created using kernel_thread() while running in the context of > some user task that entered the kernel due to a trap or syscall -- > though probably nobody should be doing that.) Thanks! Masa
On Mon, Sep 30, 2019 at 10:29:53AM -0400, Masayoshi Mizuma wrote: > Hi Julien and Dave, > > On Mon, Sep 30, 2019 at 02:02:46PM +0100, Dave Martin wrote: > > On Mon, Sep 30, 2019 at 01:23:18PM +0100, Julien Grall wrote: > > > Hi, > > > > > > On 27/09/2019 16:39, Masayoshi Mizuma wrote: > > > >From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > > > > > > >The system which has SVE feature crashed because of > > > >the memory pointed by task->thread.sve_state was destroyed > > > >by someone. > > > > > > > >That is because sve_state is freed while the forking the > > > >child process. The child process has the pointer of sve_state > > > >which is same as the parent's because the child's task_struct > > > >is copied from the parent's one. If the copy_process() > > > >fails as an error on somewhere, for example, copy_creds(), > > > >then the sve_state is freed even if the parent is alive. > > > >The flow is as follows. > > > > > > > >copy_process > > > > p = dup_task_struct > > > > => arch_dup_task_struct > > > > *dst = *src; // copy the entire region. > > > >: > > > > retval = copy_creds > > > > if (retval < 0) > > > > goto bad_fork_free; > > > >: > > > >bad_fork_free: > > > >... > > > > delayed_free_task(p); > > > > => free_task > > > > => arch_release_task_struct > > > > => fpsimd_release_task > > > > => __sve_free > > > > => kfree(task->thread.sve_state); > > > > // free the parent's sve_state > > > > > > > >Move child's sve_state = NULL and clearing TIF_SVE flag > > > >to arch_dup_task_struct() so that the child doesn't free the > > > >parent's one. > > > > > > > >Cc: stable@vger.kernel.org > > > >Fixes: bc0ee4760364 ("arm64/sve: Core task context handling") > > > > > > Looking at the log, it looks like THREAD_INFO_IN_TASK was selected before > > > the bc0ee4760364. So it should be fine to backport for all the Linux tree > > > contain this commit. > > I think this patch is needed for the kernel has SVE support. > I'll add the Cc tag as Dave said: > > Cc: stable@vger.kernel.org # 4.15+ Based on running git log --grep='stable@vger.*#.*-', the most common syntax for this situation seems to be Cc: <stable@vger.kernel.org> # 4.15.x- > So, I suppose this patch will be backported to stables 5.3.X, > 5.2.X and longterm 4.19.X. > Does this make sense? Yes, I think so. It's up to the stable maintainers to decide which trees the patch actually gets applied too. > > > >Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com> > > > >Reported-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> > > > >Suggested-by: Dave Martin <Dave.Martin@arm.com> > > > > > > I have tested the patch and can confirm that double-free disappeared after > > > the patch is applied: > > > > > > Tested-by: Julien Grall <julien.grall@arm.com> > > Thank you so much! > > > > > Good to have that confirmed -- thanks for verifying. > > > > [...] > > > > > >--- > > > > arch/arm64/kernel/process.c | 21 ++++----------------- > > > > 1 file changed, 4 insertions(+), 17 deletions(-) > > > > > > > >diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > > > >index f674f28df..6937f5935 100644 > > > >--- a/arch/arm64/kernel/process.c > > > >+++ b/arch/arm64/kernel/process.c > > > >@@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) > > > > fpsimd_release_task(tsk); > > > > } > > > >-/* > > > >- * src and dst may temporarily have aliased sve_state after task_struct > > > >- * is copied. We cannot fix this properly here, because src may have > > > >- * live SVE state and dst's thread_info may not exist yet, so tweaking > > > >- * either src's or dst's TIF_SVE is not safe. > > > >- * > > > >- * The unaliasing is done in copy_thread() instead. This works because > > > >- * dst is not schedulable or traceable until both of these functions > > > >- * have been called. > > > >- */ > > > > > > It would be good to explain in the commit message why tweaking "dst" in > > > arch_dup_task_struct() is fine. > > > > > > From my understanding, Arm64 used to have thread_info on the stack. So it > > > would not be possible to clear TIF_SVE until the stack is initialized. > > > > > > Now that the thread_info is part of the task, it should be valid to modify > > > the flag from arch_dup_task_struct(). > > > > > > Note that technically, TIF_SVE does not need to be cleared from > > > arch_dup_task_struct(). It could also be done from copy_thread(). But it is > > > easier to keep the both changes together. > > Thanks, let me add some comments to the commit log. > > > > > > > > int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) > > > > { > > > > if (current->mm) > > > > fpsimd_preserve_current_state(); > > > > *dst = *src; > > > > Ack, some more explanation would be a good idea here. > > > > Maybe the following comments are sufficient? > > > > /* We rely on the above assingment to initialise dst's thread_flags: */ > > Thanks, I'll add this comment. > > > > > > >+ BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); > > > > > > > and > > > > /* > > * Detach src's sve_state (if any) from dst so that it does not > > * get erroneously used or freed prematurely. dst's sve_state > > * will be allocated on demand later on if dst uses SVE. > > * For consistency, also clear TIF_SVE here: this could be done > > * later in copy_process(), but to avoid tripping up future > > * maintainers it is best not to leave TIF_SVE and sve_state in > > * an inconsistent state, even temporarily. > > */ > > I'll add this comments. OK, if these make sense to you, feel free to add them. [...] Cheers ---Dave
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index f674f28df..6937f5935 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -323,22 +323,16 @@ void arch_release_task_struct(struct task_struct *tsk) fpsimd_release_task(tsk); } -/* - * src and dst may temporarily have aliased sve_state after task_struct - * is copied. We cannot fix this properly here, because src may have - * live SVE state and dst's thread_info may not exist yet, so tweaking - * either src's or dst's TIF_SVE is not safe. - * - * The unaliasing is done in copy_thread() instead. This works because - * dst is not schedulable or traceable until both of these functions - * have been called. - */ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) { if (current->mm) fpsimd_preserve_current_state(); *dst = *src; + BUILD_BUG_ON(!IS_ENABLED(CONFIG_THREAD_INFO_IN_TASK)); + dst->thread.sve_state = NULL; + clear_tsk_thread_flag(dst, TIF_SVE); + return 0; } @@ -351,13 +345,6 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context)); - /* - * Unalias p->thread.sve_state (if any) from the parent task - * and disable discard SVE state for p: - */ - clear_tsk_thread_flag(p, TIF_SVE); - p->thread.sve_state = NULL; - /* * In case p was allocated the same task_struct pointer as some * other recently-exited task, make sure p is disassociated from