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SubjectRe: [PATCH v2 bpf-next 05/21] bpf: Introduce bpf_sysctl_{get,set}_new_value helpers
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On 03/26/2019 01:43 AM, Andrey Ignatov wrote:
> Add helpers to work with new value being written to sysctl by user
> space.
>
> bpf_sysctl_get_new_value() copies value being written to sysctl into
> provided buffer.
>
> bpf_sysctl_set_new_value() overrides new value being written by user
> space with a one from provided buffer. Buffer should contain string
> representation of the value, similar to what can be seen in /proc/sys/.
>
> Both helpers can be used only on sysctl write.
>
> File position matters and can be managed by an interface that will be
> introduced separately. E.g. if user space calls sys_write to a file in
> /proc/sys/ at file position = X, where X > 0, then the value set by
> bpf_sysctl_set_new_value() will be written starting from X. If program
> wants to override whole value with specified buffer, file position has
> to be set to zero.
>
> Documentation for the new helpers is provided in bpf.h UAPI.
>
> Signed-off-by: Andrey Ignatov <rdna@fb.com>
> ---
> fs/proc/proc_sysctl.c | 22 ++++++++---
> include/linux/bpf-cgroup.h | 8 ++--
> include/linux/filter.h | 3 ++
> include/uapi/linux/bpf.h | 38 +++++++++++++++++-
> kernel/bpf/cgroup.c | 81 +++++++++++++++++++++++++++++++++++++-
> 5 files changed, 142 insertions(+), 10 deletions(-)
>
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 72f4a096c146..4d1ab22774f7 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -570,8 +570,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
> struct inode *inode = file_inode(filp);
> struct ctl_table_header *head = grab_header(inode);
> struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> + void *new_buf = NULL;
> ssize_t error;
> - size_t res;
>
> if (IS_ERR(head))
> return PTR_ERR(head);
> @@ -589,15 +589,27 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
> if (!table->proc_handler)
> goto out;
>
> - error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write);
> + error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, buf, &count,
> + &new_buf);
> if (error)
> goto out;
>
> /* careful: calling conventions are nasty here */
> - res = count;
> - error = table->proc_handler(table, write, buf, &res, ppos);
> + if (new_buf) {
> + mm_segment_t old_fs;
> +
> + old_fs = get_fs();
> + set_fs(KERNEL_DS);
> + error = table->proc_handler(table, write, (void __user *)new_buf,
> + &count, ppos);
> + set_fs(old_fs);

From quick glance on the set, the above stood out. Afaik, there is an ongoing
effort by Al and other fs/core folks (as visible in the git log) to get rid of
set_fs() calls in the tree with the goal of eliminating this interface /entirely/
(more context on 'why' here: https://lwn.net/Articles/722267/). Is there a better
way to achieve the above w/o needing it?

> + kfree(new_buf);
> + } else {
> + error = table->proc_handler(table, write, buf, &count, ppos);
> + }
> +
> if (!error)
> - error = res;
> + error = count;
> out:
> sysctl_head_finish(head);
>
> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> index b1c45da20a26..1e97271f9a10 100644
> --- a/include/linux/bpf-cgroup.h
> +++ b/include/linux/bpf-cgroup.h
> @@ -113,7 +113,8 @@ int __cgroup_bpf_check_dev_permission(short dev_type, u32 major, u32 minor,
>
> int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
> struct ctl_table *table, int write,
> - enum bpf_attach_type type);
> + void __user *buf, size_t *pcount,
> + void **new_buf, enum bpf_attach_type type);
>
> static inline enum bpf_cgroup_storage_type cgroup_storage_type(
> struct bpf_map *map)
> @@ -261,11 +262,12 @@ int bpf_percpu_cgroup_storage_update(struct bpf_map *map, void *key,
> })
>
>
> -#define BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write) \
> +#define BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, buf, count, nbuf) \
> ({ \
> int __ret = 0; \
> if (cgroup_bpf_enabled) \
> __ret = __cgroup_bpf_run_filter_sysctl(head, table, write, \
> + buf, count, nbuf, \
> BPF_CGROUP_SYSCTL); \
> __ret; \
> })
> @@ -338,7 +340,7 @@ static inline int bpf_percpu_cgroup_storage_update(struct bpf_map *map,
> #define BPF_CGROUP_RUN_PROG_UDP6_SENDMSG_LOCK(sk, uaddr, t_ctx) ({ 0; })
> #define BPF_CGROUP_RUN_PROG_SOCK_OPS(sock_ops) ({ 0; })
> #define BPF_CGROUP_RUN_PROG_DEVICE_CGROUP(type,major,minor,access) ({ 0; })
> -#define BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write) ({ 0; })
> +#define BPF_CGROUP_RUN_PROG_SYSCTL(head,table,write,buf,count,nbuf) ({ 0; })
>
> #define for_each_cgroup_storage_type(stype) for (; false; )
>

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