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SubjectRe: [PATCH] [4/9] SYSCTL: Use RCU strings for core_pattern sysctl
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Andi Kleen <andi@firstfloor.org> writes:

> Also saves ~220 bytes in the data segment for default kernels.
>
> As a bonus this removes one use of the BKL.
>
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
>
> ---
> fs/exec.c | 11 +++++------
> kernel/sysctl.c | 6 +++---
> 2 files changed, 8 insertions(+), 9 deletions(-)
>
> Index: linux-2.6.33-rc2-ak/fs/exec.c
> ===================================================================
> --- linux-2.6.33-rc2-ak.orig/fs/exec.c
> +++ linux-2.6.33-rc2-ak/fs/exec.c
> @@ -62,7 +62,7 @@
> #include "internal.h"
>
> int core_uses_pid;
> -char core_pattern[CORENAME_MAX_SIZE] = "core";
> +char *core_pattern = "core";
> unsigned int core_pipe_limit;
> int suid_dumpable = 0;
>
> @@ -1421,7 +1421,7 @@ EXPORT_SYMBOL(set_binfmt);
> static int format_corename(char *corename, long signr)
> {
> const struct cred *cred = current_cred();
> - const char *pat_ptr = core_pattern;
> + const char *pat_ptr = rcu_dereference(core_pattern);

rcu_dereference should aways be between rcu_read_lock()
and rcu_read_unlock();

> int ispipe = (*pat_ptr == '|');
> char *out_ptr = corename;
> char *const out_end = corename + CORENAME_MAX_SIZE;
> @@ -1825,12 +1825,11 @@ void do_coredump(long signr, int exit_co
> clear_thread_flag(TIF_SIGPENDING);
>
> /*
> - * lock_kernel() because format_corename() is controlled by sysctl, which
> - * uses lock_kernel()
> + * Protect corename by RCU vs proc_rcu_string()
> */
> - lock_kernel();
> + rcu_read_lock();
> ispipe = format_corename(corename, signr);
> - unlock_kernel();
> + rcu_read_unlock();
>
> if ((!ispipe) && (cprm.limit < binfmt->min_coredump))
> goto fail_unlock;
> Index: linux-2.6.33-rc2-ak/kernel/sysctl.c
> ===================================================================
> --- linux-2.6.33-rc2-ak.orig/kernel/sysctl.c
> +++ linux-2.6.33-rc2-ak/kernel/sysctl.c
> @@ -75,7 +75,7 @@ extern int sysctl_oom_dump_tasks;
> extern int max_threads;
> extern int core_uses_pid;
> extern int suid_dumpable;
> -extern char core_pattern[];
> +extern char *core_pattern;
> extern unsigned int core_pipe_limit;
> extern int pid_max;
> extern int min_free_kbytes;
> @@ -399,10 +399,10 @@ static struct ctl_table kern_table[] = {
> },
> {
> .procname = "core_pattern",
> - .data = core_pattern,
> + .data = &core_pattern,
> .maxlen = CORENAME_MAX_SIZE,
> .mode = 0644,
> - .proc_handler = proc_dostring,
> + .proc_handler = proc_rcu_string,
> },
> {
> .procname = "core_pipe_limit",


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