[PATCH v3] Print absolute paths in printpathn when -yy is used

[Mike Frysinger]

On Sun 15 Jun 2014 09:14:05 zubin.mithra at gmail.com wrote:
> From: Zubin Mithra <zubin.mithra at gmail.com>
> 
> * defs.h (show_fd_path): Change type to unsigned int.
> * strace.c (show_fd_path): Update usage to count y flag.
> * util.c (print_tracee_cwd): New function.
> (printpathn): Update to use print_tracee_cwd and print
>  absolute path.
> * strace.1: Add description of -yy option

you should also update the usage string so `strace -h` documents -yy

> +int
> +print_tracee_cwd(struct tcb *tcp)
> +{
> +	int link_size = sizeof("/proc/%u/cwd") + sizeof(int) * 3;
> +	char linkpath[link_size];
> +	char cwd[MAXPATHLEN+2];

there should be spaces around that +

> +	ssize_t n;
> +
> +	snprintf(linkpath, link_size, "/proc/%u/cwd", tcp->pid);
> +	n = readlink(linkpath, cwd, MAXPATHLEN);

anytime you start using magic constant lengths for paths, you know you're 
probably doing something wrong.  you have to remember that some systems don't 
have a max path length restriction so these constants won't even be defined.

if you lstat() the path, the st_size will tell you the length, however that 
has a TOCTOU race (another thread could change the working dir), so i guess 
that's not really viable :(.

you could use an alloca() buffer starting at a "large enough" value to cover 
most common cases (like 256) and put it into a loop -- when the return value 
of readlink is equal to bufsiz, double the buffer length and try again.

i would also highlight that some systems (really just nommu) really do not 
want large buffer stacks because the stack size is fixed.  by putting large 
values there, you can easily blow it (which this code is doing).  if this 
particular func matters to performance, we can use a cached malloc buffer 
(that you can realloc() to increase over time).
-mike
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