On Mon, Mar 14, 2016 at 02:27:56PM -0400, Mike Frysinger wrote:
> On 14 Mar 2016 18:11, Bartosz Gołaszewski wrote:
> > 2016-03-14 15:27 GMT+01:00 Mike Frysinger <[email protected]>:
> > > On 14 Mar 2016 11:07, Bartosz Golaszewski wrote:
> > >> +#ifndef __BB_NAMESPACE_H
> > >> +#define __BB_NAMESPACE_H
> > >
> > > use a naming style like other busybox headers
> > >
> > >> +/*
> > >> + * Longest possible path to a procfs file used in namespace utils. Must
> > >> be
> > >> + * able to contain the '/proc/' string, the '/ns/user' string which is
> > >> the
> > >> + * longest namespace name and a 32-bit integer representing the process
> > >> ID.
> > >> + */
> > >> +#define NS_PROC_PATH_MAX (sizeof("/proc//ns/user") + sizeof(pid_t) * 3)
> > >
> > > using the sizeof pid_t as a proxy for how many chars it'd take to render
> > > a decimal number in ASCII is wonky. just hardcode it as "10" since that
> > > is the largest unsigned 32bit number ("4294967296").
> >
> > Can you elaborate on how it's wonky? While your solution is perfectly
> > fine I think that there's nothing wrong with the way I've done it
> > neither.
>
> the code seems to assume that the byte size scales into the number of
> chars required to represent the number in base 10 when it's really a
> log scale. here's a table to show it's wonky:
sizeof is already a log scale. 3*sizeof(pid_t) is
3*log_256(MAX_PID+1), which is greater than 3*ceil(log_1000(MAX_PID)),
the actual space requirement.
> pid_t |sizeof |*3 val|actual|
> size |(bytes)|(char)| |
> (bits)| | | |
> ------|-------|------|------|
> 8 | 1 | 3 | 4 |
> 16 | 2 | 6 | 6 |
> 32 | 4 | 12 | 11 |
> 64 | 8 | 24 | 20 |
> 128 | 16 | 48 | 40 |
>
> the "actual" value is one higher than you might expect because pid_t
> is a signed quantity. for 8bit, "-128" is 0x80 and takes 4 bytes.
> for 32bit ("int"), "-2147483648" is 0x80000000 and takes 11 bytes.
While pid_t is signed, negative values are not meaningful as pids;
they're pgids or special sentinels in some contexts. If this code is
trying to print a negative pid into a pathname used in /proc, that's a
bug. But in general, when using the 3*sizeof idiom you should add +1
for a sign if needed and another +1 for null termination if not
already included elsewhere.
Rich
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