Re: [PATCH] linux-user: Fix openat() emulation to correctly detect accesses to /proc

[Daniel P . Berrangé]

On Tue, Aug 01, 2023 at 04:58:57PM +0200, Helge Deller wrote:
> In qemu we catch accesses to files like /proc/cpuinfo or /proc/net/route
> and return to the guest contents which would be visible on a real system
> (instead what the host would show).
> 
> This patch fixes a bug, where for example the accesses
>     cat /proc////cpuinfo
> or
>     cd /proc && cat cpuinfo
> will not be recognized by qemu and where qemu will wrongly show
> the contents of the host's /proc/cpuinfo file.
> 
> Signed-off-by: Helge Deller <deller@gmx.de>
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 917c388073..bb864c2bb3 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8531,9 +8531,11 @@ static int open_cpuinfo(CPUArchState *cpu_env, int fd)
>  }
>  #endif
> 
> -int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *pathname,
> +int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
>                      int flags, mode_t mode, bool safe)
>  {
> +    char proc_name[PATH_MAX];

No PATH_MAX buffers declared on the stack please.

> +    const char *pathname;
>      struct fake_open {
>          const char *filename;
>          int (*fill)(CPUArchState *cpu_env, int fd);
> @@ -8560,6 +8562,13 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, 
> const char *pathname,
>          { NULL, NULL, NULL }
>      };
> 
> +    /* if this is a file from /proc/ filesystem, expand full name */
> +    if (realpath(fname, proc_name) && strncmp(proc_name, "/proc/", 6) == 0) {

QEMU relies on the extended semantics of realpath() where passing NULL
for 'proc_name' causes it to allocate a buffer of the correct size and
return the new buffer. Using that avoids PATH_MAX variables.

> +        pathname = proc_name;
> +    } else {
> +        pathname = fname;
> +    }

With regards,
Daniel
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