Messages in this thread |  | | Date | Thu, 9 Apr 2015 21:12:00 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] fs/proc: allow larger /proc/<pid>/cmdline output |
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On Thu, 9 Apr 2015 23:59:02 -0400 Jarod Wilson <jarod@redhat.com> wrote:
> There are people who run java. Sometimes, when it misbehaves, they try to > figure out what's going on by dumping /proc/<pid>/cmdline, but the length > of that output is currently capped by PAGE_SIZE (so x86_64's 4k, in most > cases), and sometimes, java command lines are longer than 4k characters. > > This change allows the user to request a larger max length, up to 4x > PAGE_SIZE, but the default out-of-the-box setting should keep things the > same as they ever were. The 4x maximum is somewhat arbitrary, but seemed > like it should be more than enough, because really, if you have more than > 16k characters on your command line, you're probably doing it wrong... > > I've tested this lightly with non-java shell commands with really long > parameters, and things are perfectly stable after several hundred > iterations of exercising things on a system booted with both > proc_pid_maxlen=8192 and 16384. I wouldn't call my testing exhaustive, > and I may not have considered something that will blow up horribly here, > so comments and clues welcomed. > > Using single_open_size() looked less messy than giving proc_pid_cmdline() > its own .start op that would allow multiple buffers. > > Note: I've only added this extended sizing for /proc/<pid>/cmdline output, > rather than for all /proc/<pid>/foo elements, thinking that nothing else > should ever really be that long, but anything that is can simply switch > from using the ONE() macro to the ONE_SIZE() macro.
Why have an upper limit at all?
> --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -134,6 +134,30 @@ struct pid_entry { > NOD(NAME, (S_IFREG|(MODE)), \ > NULL, &proc_single_file_operations, \ > { .proc_show = show } ) > +#define ONE_SIZE(NAME, MODE, show) \ > + NOD(NAME, (S_IFREG|(MODE)), \ > + NULL, &proc_single_file_size_operations, \ > + { .proc_show = show } ) > + > +/* > + * Its hideous, but some java gunk winds up with a cmdline that is longer > + * than PAGE_SIZE, and some people want to be able to see all of it for > + * debugging purposes. Allocate at least PAGE_SIZE, and allow the user to > + * ask for up to PAGE_SIZE << 2 (4x) to help with that situation. > + */ > +static unsigned long proc_pid_maxlen = PAGE_SIZE; > +static int set_proc_pid_maxlen(char *str) > +{ > + if (!str) > + return 0; > + > + proc_pid_maxlen = simple_strtoul(str, &str, 0); > + proc_pid_maxlen = max(PAGE_SIZE, proc_pid_maxlen); > + proc_pid_maxlen = min(PAGE_SIZE << 2, proc_pid_maxlen); > + > + return 1; > +} > +__setup("proc_pid_maxlen=", set_proc_pid_maxlen);
This permits 4k-16k on x86 and 64k-256k on powerpc. This makes the kernel interface inconsistent across architectures, which is not good - some applications will work OK on one arch but will fail when moved to a different arch.
s/PAGE_SIZE/4096/g would fix that.
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