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Re: [PATCH v3 1/2] use HOST_NAME_MAX to size buffers for gethostname(2)




Hi,

David Turner wrote:

> From: René Scharfe <l.s.r@xxxxxx>
>
> POSIX limits the length of host names to HOST_NAME_MAX.  Export the
> fallback definition from daemon.c and use this constant to make all
> buffers used with gethostname(2) big enough for any possible result
> and a terminating NUL.

Since some platforms do not define HOST_NAME_MAX and we provide a
fallback, this is not actually big enough for any possible result.
For example, the Hurd allows arbitrarily long hostnames.

Nevertheless this patch seems like the right thing to do.

> Inspired-by: David Turner <dturner@xxxxxxxxxxxx>
> Signed-off-by: Rene Scharfe <l.s.r@xxxxxx>
> Signed-off-by: David Turner <dturner@xxxxxxxxxxxx>
> ---
>  builtin/gc.c           | 10 +++++++---
>  builtin/receive-pack.c |  2 +-
>  daemon.c               |  4 ----
>  fetch-pack.c           |  2 +-
>  git-compat-util.h      |  4 ++++
>  ident.c                |  2 +-
>  6 files changed, 14 insertions(+), 10 deletions(-)

Thanks for picking this up.

[...]
> +++ b/builtin/gc.c
[...]
> @@ -257,8 +257,12 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
>  	fd = hold_lock_file_for_update(&lock, pidfile_path,
>  				       LOCK_DIE_ON_ERROR);
>  	if (!force) {
> -		static char locking_host[128];
> +		static char locking_host[HOST_NAME_MAX + 1];
> +		static char *scan_fmt;
>  		int should_exit;
> +
> +		if (!scan_fmt)
> +			scan_fmt = xstrfmt("%s %%%dc", "%"SCNuMAX, HOST_NAME_MAX);
>  		fp = fopen(pidfile_path, "r");
>  		memset(locking_host, 0, sizeof(locking_host));
>  		should_exit =
> @@ -274,7 +278,7 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
>  			 * running.
>  			 */
>  			time(NULL) - st.st_mtime <= 12 * 3600 &&
> -			fscanf(fp, "%"SCNuMAX" %127c", &pid, locking_host) == 2 &&
> +			fscanf(fp, scan_fmt, &pid, locking_host) == 2 &&

I hoped this could be simplified since HOST_NAME_MAX is a numeric literal,
using the double-expansion trick:

#define STR_(s) # s
#define STR(s) STR_(s)

			fscanf(fp, "%" SCNuMAX " %" STR(HOST_NAME_MAX) "c",
			       &pid, locking_host);

Unfortunately, I don't think there's anything stopping a platform from
defining

	#define HOST_NAME_MAX 0x100

which would break that.

So this run-time calculation appears to be necessary.

Reviewed-by: Jonathan Nieder <jrnieder@xxxxxxxxx>

Thanks.