[PATCH v5 4/4] Add system-wide tunables: Filters

[Florian Weimer]

* DJ Delorie:

> @@ -340,9 +349,58 @@ __tunables_init (char **envp)
>  	      if (tid == -1)
>  		continue;
>  	    }
> -	  /* At this point, TID is valid for the tunable we want.  See
> -	     if the parsed type matches the desired type.  */
> -
> +	  /* At this point, TID is valid for the tunable we want.  */
> +
> +	  /* Apply selected filter, if any.  */
> +	  switch (tec->flags & TUNCONF_FLAG_FILTER) {
> +	  case TUNCONF_FILTER_PERPROC:
> +	    /* Perform one-time calculations that aren't needed if we
> +	       don't use this filter.  */
> +	    if (prog_name_len == -1)
> +	      {
> +		ssize_t n = readlink ("/proc/self/exe",
> +				      exebuf, sizeof (exebuf) - 1);
> +		if (n > 0 && n < sizeof(exebuf)-1)
> +		  {
> +		    /* If /proc/self/exe exists and we can read it,
> +		       it's more reliable than argv[] so use it.  */
> +		    exebuf[n] = '\0';
> +		    prog_name = exebuf;
> +		  }
> +		if (prog_name != NULL)
> +		  {
> +		    const char *slash = NULL, *cp;
> +		    for (cp = prog_name; *cp; ++ cp)
> +		      if (*cp == '/')
> +			slash = cp;
> +		    if (slash)
> +		      base_name = slash + 1;
> +		    else
> +		      base_name = prog_name;
> +		    prog_name_len = strlen (prog_name);
> +		    base_name_len = strlen (base_name);
> +		  }

Ideally this would be consolidated with the l_origin computation for the
main map. The logic seems a bit backwards to me.

I think we need two matching modes here:

* The path is deemed trusted by the system administrator.  The tunable
  is used for all binaries (including when __libc_enable_secure is
  active).

* The path is not trusted.  The tunable is not used in AT_SECURE mode.

All argv[0] comparisons are not trusted (not used for AT_SECURE mode).

In any case, this needs more documentation about the syntax.

> diff --git a/elf/tst-tunconf1.root/etc/tunables.conf b/elf/tst-tunconf1.root/etc/tunables.conf
> new file mode 100644
> index 0000000000..3c7b33c399
> --- /dev/null
> +++ b/elf/tst-tunconf1.root/etc/tunables.conf
> @@ -0,0 +1,6 @@
> +glibc.malloc.tcache_max=6
> +glibc.malloc.tcache_count=3
> +[proc:/bin/ls]
> +glibc.malloc.tcache_max=7
> +[proc:tst-tunconf1]
> +glibc.malloc.tcache_max=4

I'm still a bit on the fence regarding the separate tunables.conf file.
I think deployment (through tools like Ansible) is easier if integrated
with the existing ld.so.conf framework.  I would recommend a more
keyword-oriented syntax that allows future introduction of additional
directives.  So perhaps something like:

  match-executable-path "/usr/bin/ls"
  set-tunable glibc.malloc.tcache_max=7

We should have a proper way for quoting strings from the beginning.

Thanks,
Florian