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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Tue, 1 Mar 2016 12:35:18 +0100
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Subject: Re: [Qemu-devel] [PATCH 01/38] exec: Fix memory allocation when
 memory path names new file
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To: Markus Armbruster <armbru@redhat.com>, qemu-devel@nongnu.org
Cc: mlureau@redhat.com, cam@cs.ualberta.ca, claudio.fontana@huawei.com, david.marchand@6wind.com



On 29/02/2016 19:40, Markus Armbruster wrote:
> -    if (!stat(path, &st) && S_ISDIR(st.st_mode)) {
> +    ret = stat(path, &st);
> +    if (!ret && S_ISDIR(st.st_mode)) {
> +        /* path names a directory -> create a temporary file there */
>          /* Make name safe to use with mkstemp by replacing '/' with '_'. */
>          sanitized_name = g_strdup(memory_region_name(block->mr));
>          for (c = sanitized_name; *c != '\0'; c++) {
> @@ -1282,13 +1271,32 @@ static void *file_ram_alloc(RAMBlock *block,
>              unlink(filename);
>          }
>          g_free(filename);
> +    } else if (!ret) {
> +        /* path names an existing file -> use it */
> +        fd = open(path, O_RDWR);
>      } else {
> +        /* create a new file */
>          fd = open(path, O_RDWR | O_CREAT, 0644);
> +        unlink_on_error = true;
>      }

While at it, let's avoid TOCTTOU conditions:

    for (;;) {
        fd = open(path, O_RDWR);
        if (fd != -1) {
            break;
        }
        if (errno == ENOENT) {
            fd = open(path, O_RDWR | O_CREAT | O_EXCL, 0644);
            if (fd != -1) {
                unlink_on_error = true;
                break;
            }
        } else if (errno == EISDIR) {
            ... mkstemp ...
            if (fd != -1) {
                unlink_on_error = true;
                break;
            }
        }
        if (errno != EEXIST && errno != EINTR) {
            goto error;
        }
    }

and use fstatfs in gethugepagesize.

Paolo