Re: [PATCH] perf tools: Restore proper cwd on return from mnt ns

[Arnaldo Carvalho de Melo]

Em Thu, Nov 01, 2018 at 06:00:01PM +0100, Jiri Olsa escreveu:
> When reporting on 'record' server we try to retrieve/use
> the mnt namespace of the profiled tasks. We use following
> API with cookie to hold the return namespace, roughly:
> 
>   nsinfo__mountns_enter(struct nsinfo *nsi, struct nscookie *nc)
>     setns(newns, 0);
>   ...
>   new ns related open..
>   ...
>   nsinfo__mountns_exit(struct nscookie *nc)
>     setns(nc->oldns)
> 
> Once finished we setns to old namespace, which also sets the
> current working directory (cwd) to "/", trashing the cwd we had.
> 
> This is mostly fine, because we use absolute paths almost everywhere,
> but it screws up perf diff:
> 
>   # perf diff
>   failed to open perf.data: No such file or directory  (try 'perf record' first)
>   ...
> 
> Adding the current working directory to be part of the cookie
> and restoring it in the nsinfo__mountns_exit call.
 
> Cc: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
> Fixes: 843ff37bb59e ("perf symbols: Find symbols in different mount namespace")
> Link: http://lkml.kernel.org/n/tip-zg3vz7kjr86cco7lo91v8yhj@xxxxxxxxxxxxxx
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  tools/perf/util/namespaces.c | 18 ++++++++++++++++--
>  tools/perf/util/namespaces.h |  1 +
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/namespaces.c b/tools/perf/util/namespaces.c
> index cf8bd123cf73..fb0458b7e6aa 100644
> --- a/tools/perf/util/namespaces.c
> +++ b/tools/perf/util/namespaces.c
> @@ -18,6 +18,7 @@
>  #include <stdio.h>
>  #include <string.h>
>  #include <unistd.h>
> +#include <asm/bug.h>
>  
>  struct namespaces *namespaces__new(struct namespaces_event *event)
>  {
> @@ -186,6 +187,7 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
>  	char curpath[PATH_MAX];
>  	int oldns = -1;
>  	int newns = -1;
> +	char *oldcwd = NULL;
>  
>  	if (nc == NULL)
>  		return;
> @@ -199,9 +201,13 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
>  	if (snprintf(curpath, PATH_MAX, "/proc/self/ns/mnt") >= PATH_MAX)
>  		return;
>  
> +	oldcwd = get_current_dir_name();
> +	if (!oldcwd)
> +		return;
> +
>  	oldns = open(curpath, O_RDONLY);
>  	if (oldns < 0)
> -		return;
> +		goto errout;
>  
>  	newns = open(nsi->mntns_path, O_RDONLY);
>  	if (newns < 0)
> @@ -210,11 +216,14 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
>  	if (setns(newns, CLONE_NEWNS) < 0)
>  		goto errout;
>  
> +	nc->oldcwd = oldcwd;
>  	nc->oldns = oldns;
>  	nc->newns = newns;
>  	return;
>  
>  errout:
> +	if (oldcwd)
> +		free(oldcwd);

Applied, and while at it I removed the needless if in the above two
lines.

>  	if (oldns > -1)
>  		close(oldns);
>  	if (newns > -1)
> @@ -223,11 +232,16 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
>  
>  void nsinfo__mountns_exit(struct nscookie *nc)
>  {
> -	if (nc == NULL || nc->oldns == -1 || nc->newns == -1)
> +	if (nc == NULL || nc->oldns == -1 || nc->newns == -1 || !nc->oldcwd)
>  		return;
>  
>  	setns(nc->oldns, CLONE_NEWNS);
>  
> +	if (nc->oldcwd) {
> +		WARN_ON_ONCE(chdir(nc->oldcwd));
> +		free(nc->oldcwd);
> +	}
> +
>  	if (nc->oldns > -1) {
>  		close(nc->oldns);
>  		nc->oldns = -1;
> diff --git a/tools/perf/util/namespaces.h b/tools/perf/util/namespaces.h
> index cae1a9a39722..d5f46c09ea31 100644
> --- a/tools/perf/util/namespaces.h
> +++ b/tools/perf/util/namespaces.h
> @@ -38,6 +38,7 @@ struct nsinfo {
>  struct nscookie {
>  	int			oldns;
>  	int			newns;
> +	char			*oldcwd;
>  };
>  
>  int nsinfo__init(struct nsinfo *nsi);
> -- 
> 2.17.2