On Tue, Jan 07, 2025 at 04:33:40PM +0100, Gerd Hoffmann wrote:
> Define qapi schema for the uefi variable store state.
>
> Use it and the generated visitor helper functions to store persistent
> (EFI_VARIABLE_NON_VOLATILE) variables in JSON format on disk.
>
> Signed-off-by: Gerd Hoffmann <[email protected]>
> ---
> hw/uefi/var-service-json.c | 242 +++++++++++++++++++++++++++++++++++++
> qapi/meson.build | 1 +
> qapi/qapi-schema.json | 1 +
> qapi/uefi.json | 45 +++++++
> 4 files changed, 289 insertions(+)
> create mode 100644 hw/uefi/var-service-json.c
> create mode 100644 qapi/uefi.json
>
> +void uefi_vars_json_init(uefi_vars_state *uv, Error **errp)
> +{
> + if (uv->jsonfile) {
> + uv->jsonfd = qemu_create(uv->jsonfile, O_RDWR, 0666, errp);
> + }
> +}
> +
> +void uefi_vars_json_save(uefi_vars_state *uv)
> +{
> + GString *gstr;
> + int rc;
> +
> + if (uv->jsonfd == -1) {
> + return;
> + }
> +
> + gstr = uefi_vars_to_json(uv);
> +
> + lseek(uv->jsonfd, 0, SEEK_SET);
> + rc = write(uv->jsonfd, gstr->str, gstr->len);
> + if (rc != gstr->len) {
> + warn_report("%s: write error", __func__);
> + }
> + rc = ftruncate(uv->jsonfd, gstr->len);
> + if (rc != 0) {
> + warn_report("%s: ftruncate error", __func__);
> + }
> + fsync(uv->jsonfd);
Although the fsync helps, re-writing the file in-place is a ad idea for data
integrity on host OS crash. Especially if the new data is shorter, we would
easily end up with a file containing old and new data making it unparsable
(assuming the parser doesn't ignore trailing data).
I'd like to suggest the write to temp file + rename dance to get atomic
replacement. The problem with that is that it hits the DAC/MAC security
restrictions we put QEMU under :-(
My next best idea is to re-arrange things thus:
lseek(fd, 0)
ftruncate(fd, 0)
fsync(fd)
write(fd, str)
fsync(fd)
so we should at least reduce the liklihood of getting a mix of
old and new data - empty file is better than a mix of data.
With regards,
Daniel
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