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Re: [Qemu-devel] [PATCH v2 4/5] fw_cfg: prohibit insertion of duplicate

From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH v2 4/5] fw_cfg: prohibit insertion of duplicate fw_cfg file names
Date: Fri, 20 Mar 2015 07:51:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 03/19/15 01:18, Gabriel L. Somlo wrote:
> Exit with an error (instead of simply logging a trace event)
> whenever the same fw_cfg file name is added multiple times via
> one of the fw_cfg_add_file[_callback]() host-side API calls.
> Signed-off-by: Gabriel Somlo <address@hidden>
> ---
>  hw/nvram/fw_cfg.c | 11 ++++++-----
>  trace-events      |  1 -
>  2 files changed, 6 insertions(+), 6 deletions(-)
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 659de4c..a5fd512 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -505,18 +505,19 @@ void fw_cfg_add_file_callback(FWCfgState *s,  const 
> char *filename,
>      index = be32_to_cpu(s->files->count);
>      assert(index < FW_CFG_FILE_SLOTS);
> -    fw_cfg_add_bytes_read_callback(s, FW_CFG_FILE_FIRST + index,
> -                                   callback, callback_opaque, data, len);
> -
>      pstrcpy(s->files->f[index].name, sizeof(s->files->f[index].name),
>              filename);
>      for (i = 0; i < index; i++) {
>          if (strcmp(s->files->f[index].name, s->files->f[i].name) == 0) {
> -            trace_fw_cfg_add_file_dupe(s, s->files->f[index].name);
> -            return;
> +            error_report("duplicate fw_cfg file name: %s",
> +                         s->files->f[index].name);
> +            exit(1);
>          }
>      }
> +    fw_cfg_add_bytes_read_callback(s, FW_CFG_FILE_FIRST + index,
> +                                   callback, callback_opaque, data, len);
> +
>      s->files->f[index].size   = cpu_to_be32(len);
>      s->files->f[index].select = cpu_to_be16(FW_CFG_FILE_FIRST + index);
>      trace_fw_cfg_add_file(s, index, s->files->f[index].name, len);
> diff --git a/trace-events b/trace-events
> index 1275b70..a340c5a 100644
> --- a/trace-events
> +++ b/trace-events
> @@ -195,7 +195,6 @@ ecc_diag_mem_readb(uint64_t addr, uint32_t ret) "Read 
> diagnostic %"PRId64"= %02x
>  # hw/nvram/fw_cfg.c
>  fw_cfg_select(void *s, uint16_t key, int ret) "%p key %d = %d"
>  fw_cfg_read(void *s, uint8_t ret) "%p = %d"
> -fw_cfg_add_file_dupe(void *s, char *name) "%p %s"
>  fw_cfg_add_file(void *s, int index, char *name, size_t len) "%p #%d: %s (%zd 
> bytes)"
>  # hw/block/hd-geometry.c

Here's an idea I had this morning.

This series gives equal rank to fw_cfg file names that originate
internally and those that come from the user, via the command line.

That means that whenever qemu developers want to introduce a new fw_cfg
file, they can never be sure that the new name will not conflict with
something a user has already been passing in via the command line, for
whatever purposes. (Because, well, that's the goal of this patchset, to
empower the user to pass in fw_cfg files independently of qemu developers.)

This looks brittle. How about:

(a) advising users in the docs txt *and in the manual* to use some kind
of fw_cfg file name prefix, like "usr/" or "opt/", and then steering
clear of such prefixes in qemu, as far as developers are concerned. Or,

(b) automatically prepending "opt/" or "usr/" to all fw_cfg file names
that come via -fw_cfg (equiv. via [fw_cfg] in the config file), and, for
developers, steering clear of those prefixes in qemu's source.

The C standard and the POSIX standard define lists of identifier
prefixes (well, patterns) that are reserved for various uses. If a
program violates that, it might not compile on some platform, or with
the next release of the compiler on the same platform etc. I think we
should posit something like this.

Personally I vote (a). Document it, but don't enforce it.

(Assuming that a user-specified fw_cfg file gains traction, and becomes
popular to the point that qemu wants to expose it itself, then qemu can
just generate the same file with (eg.) an "etc/" prefix. And then
firmware (or other guest code) can start looking for the file under both
prefixes, and give priority to... well, that's another policy question;
but we're talking mechanism thus far. :))

Thoughts about (a) vs. (b) vs. neither?


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