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Howlett" , Suren Baghdasaryan , Vlastimil Babka , linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Oliver Sang , John Hubbard Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Oct 25, 2024 at 10:41=E2=80=AFAM Lorenzo Stoakes wrote: > > It is useful to be able to utilise the pidfd mechanism to reference the > current thread or process (from a userland point of view - thread group > leader from the kernel's point of view). > > Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and > PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader. > > For convenience and to avoid confusion from userland's perspective we ali= as > these: > > * PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always wh= at > the user will want to use, as they would find it surprising if for > instance fd's were unshared()'d and they wanted to invoke pidfd_getfd() > and that failed. > > * PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users > have no concept of thread groups or what a thread group leader is, and > from userland's perspective and nomenclature this is what userland > considers to be a process. > > Due to the refactoring of the central __pidfd_get_pid() function we can > implement this functionality centrally, providing the use of this sentine= l > in most functionality which utilises pidfd's. > > We need to explicitly adjust kernel_waitid_prepare() to permit this (thou= gh > it wouldn't really make sense to use this there, we provide the ability f= or > consistency). > > We explicitly disallow use of this in setns(), which would otherwise have > required explicit custom handling, as it doesn't make sense to set the > current calling thread to join the namespace of itself. > > As the callers of pidfd_get_pid() expect an increased reference count on > the pid we do so in the self case, reducing churn and avoiding any breaka= ge > from existing logic which decrements this reference count. > > This change implicitly provides PIDFD_SELF_* support in the waitid(P_PIDF= S, > ...), process_madvise(), process_mrelease(), pidfd_send_signal(), and > pidfd_getfd() system calls. > > Things such as polling a pidfs and general fd operations are not supporte= d, > this strictly provides the sentinel for APIs which explicitly accept a > pidfd. > > Reviewed-by: Shakeel Butt > Signed-off-by: Lorenzo Stoakes > --- > include/linux/pid.h | 8 ++++-- > include/uapi/linux/pidfd.h | 15 +++++++++++ > kernel/exit.c | 3 ++- > kernel/nsproxy.c | 1 + > kernel/pid.c | 51 ++++++++++++++++++++++++-------------- > 5 files changed, 57 insertions(+), 21 deletions(-) > > diff --git a/include/linux/pid.h b/include/linux/pid.h > index d466890e1b35..3b2ac7567a88 100644 > --- a/include/linux/pid.h > +++ b/include/linux/pid.h > @@ -78,11 +78,15 @@ struct file; > * __pidfd_get_pid() - Retrieve a pid associated with the specified pidf= d. > * > * @pidfd: The pidfd whose pid we want, or the fd of a /proc/ = file if > - * @alloc_proc is also set. > + * @alloc_proc is also set, or PIDFD_SELF_* to refer to the= current > + * thread or thread group leader. > * @allow_proc: If set, then an fd of a /proc/ file can be passed i= nstead > * of a pidfd, and this will be used to determine the pid. > + > * @flags: Output variable, if non-NULL, then the file->f_flags of = the > - * pidfd will be set here. > + * pidfd will be set here or If PIDFD_SELF_THREAD is set, t= his is > + * set to PIDFD_THREAD, otherwise if PIDFD_SELF_THREAD_GROU= P then > + * this is set to zero. > * > * Returns: If successful, the pid associated with the pidfd, otherwise = an > * error. > diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h > index 565fc0629fff..0ca2ebf906fd 100644 > --- a/include/uapi/linux/pidfd.h > +++ b/include/uapi/linux/pidfd.h > @@ -29,4 +29,19 @@ > #define PIDFD_GET_USER_NAMESPACE _IO(PIDFS_IOCTL_MAGIC, 9) > #define PIDFD_GET_UTS_NAMESPACE _IO(PIDFS_IOCTL_MAGIC, 10) > > +/* > + * Special sentinel values which can be used to refer to the current thr= ead or > + * thread group leader (which from a userland perspective is the process= ). > + */ > +#define PIDFD_SELF PIDFD_SELF_THREAD > +#define PIDFD_SELF_PROCESS PIDFD_SELF_THREAD_GROUP > + > +#define PIDFD_SELF_THREAD -100 /* Current thread. */ This conflicts with AT_FDCWD, might be worth changing? > +#define PIDFD_SELF_THREAD_GROUP -200 /* Current thread group lead= er. */ We might want to pick some range outside of the negative errno space (-4096 IIRC), since we have plenty of values to pick from (2^31 at least). > +static inline int pidfd_is_self_sentinel(pid_t pid) > +{ > + return pid =3D=3D PIDFD_SELF_THREAD || pid =3D=3D PIDFD_SELF_THRE= AD_GROUP; > +} Do we want this in the uapi header? Even if this is useful, it might come with several drawbacks such as breaking scripts that parse kernel headers (and a quick git grep suggests we do have static inlines in headers, but in rather obscure ones) and breaking C89: :8:8: error: unknown type name 'inline' 8 | static inline int pidfd_is_self_sentinel(pid_t pid) :) > + > #endif /* _UAPI_LINUX_PIDFD_H */ > diff --git a/kernel/exit.c b/kernel/exit.c > index 619f0014c33b..3eb20f8252ee 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -71,6 +71,7 @@ > #include > #include > > +#include > #include > > #include > @@ -1739,7 +1740,7 @@ int kernel_waitid_prepare(struct wait_opts *wo, int= which, pid_t upid, > break; > case P_PIDFD: > type =3D PIDTYPE_PID; > - if (upid < 0) > + if (upid < 0 && !pidfd_is_self_sentinel(upid)) > return -EINVAL; > > pid =3D pidfd_get_pid(upid, &f_flags); > diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c > index dc952c3b05af..d239f7eeaa1f 100644 > --- a/kernel/nsproxy.c > +++ b/kernel/nsproxy.c > @@ -550,6 +550,7 @@ SYSCALL_DEFINE2(setns, int, fd, int, flags) > struct nsset nsset =3D {}; > int err =3D 0; > > + /* If fd is PIDFD_SELF_*, implicitly fail here, as invalid. */ > if (!fd_file(f)) > return -EBADF; > > diff --git a/kernel/pid.c b/kernel/pid.c > index 94c97559e5c5..8742157b36f8 100644 > --- a/kernel/pid.c > +++ b/kernel/pid.c > @@ -535,33 +535,48 @@ struct pid *find_ge_pid(int nr, struct pid_namespac= e *ns) > } > EXPORT_SYMBOL_GPL(find_ge_pid); > > +static struct pid *pidfd_get_pid_self(unsigned int pidfd, unsigned int *= flags) > +{ > + bool is_thread =3D pidfd =3D=3D PIDFD_SELF_THREAD; > + enum pid_type type =3D is_thread ? PIDTYPE_PID : PIDTYPE_TGID; > + struct pid *pid =3D *task_pid_ptr(current, type); > + > + /* The caller expects an elevated reference count. */ > + get_pid(pid); It would be really really nice to avoid the get here, but I imagine it'll take some refactoring around put_pid's? > + return pid; > +} > + > struct pid *__pidfd_get_pid(unsigned int pidfd, bool allow_proc, > unsigned int *flags) > { > - struct pid *pid; > - struct fd f =3D fdget(pidfd); > - struct file *file =3D fd_file(f); > + if (pidfd_is_self_sentinel(pidfd)) { > + return pidfd_get_pid_self(pidfd, flags); > + } else { Skipping the else here might make the rest of the code more legible (since the sentinel branch returns anyway...). --=20 Pedro