| Message ID | 20220903162302.3176003-2-hpoussin@reactos.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Fix some problems with vvfat in R/W mode | expand |
Am 03.09.2022 um 18:23 hat Hervé Poussineau geschrieben: > 'reserved1' field in bootsector is used to mark volume dirty, or need to verify. > Allow writes to bootsector which only changes the 'reserved1' field. > > This fixes I/O errors on Windows guests. > > Resolves: https://bugs.launchpad.net/qemu/+bug/1889421 > Signed-off-by: Hervé Poussineau <hpoussin@reactos.org> > --- > block/vvfat.c | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/block/vvfat.c b/block/vvfat.c > index d6dd919683d..35057a51c67 100644 > --- a/block/vvfat.c > +++ b/block/vvfat.c > @@ -2993,11 +2993,27 @@ DLOG(checkpoint()); > > vvfat_close_current_file(s); > > + if (sector_num == s->offset_to_bootsector && nb_sectors == 1) { > + /* > + * Write on bootsector. Allow only changing the reserved1 field, > + * used to mark volume dirtiness > + */ > + const unsigned char *initial = s->first_sectors > + + s->offset_to_bootsector * 0x200; > + for (i = 0; i < 0x200; i++) { > + if (i != offsetof(bootsector_t, u.fat16.reserved1) && I think you need to check the FAT version (s->fat_type) before accessing u.fat16. For FAT32, the "reserved" field is at a different offset (but seems to have the same meaning). > + initial[i] != buf[i]) { > + fprintf(stderr, "Tried to write to protected bootsector\n"); > + return -1; > + } > + } > + return 0; > + } Should we update s->first_sectors with the new value so that the guest would actually read back what it wrote instead of having the change disappear magically? > /* > * Some sanity checks: > * - do not allow writing to the boot sector > */ > - > if (sector_num < s->offset_to_fat) > return -1; Kevin
Le 29/09/2022 à 16:10, Kevin Wolf a écrit : > Am 03.09.2022 um 18:23 hat Hervé Poussineau geschrieben: >> 'reserved1' field in bootsector is used to mark volume dirty, or need to verify. >> Allow writes to bootsector which only changes the 'reserved1' field. >> >> This fixes I/O errors on Windows guests. >> >> Resolves: https://bugs.launchpad.net/qemu/+bug/1889421 >> Signed-off-by: Hervé Poussineau <hpoussin@reactos.org> >> --- >> block/vvfat.c | 18 +++++++++++++++++- >> 1 file changed, 17 insertions(+), 1 deletion(-) >> >> diff --git a/block/vvfat.c b/block/vvfat.c >> index d6dd919683d..35057a51c67 100644 >> --- a/block/vvfat.c >> +++ b/block/vvfat.c >> @@ -2993,11 +2993,27 @@ DLOG(checkpoint()); >> >> vvfat_close_current_file(s); >> >> + if (sector_num == s->offset_to_bootsector && nb_sectors == 1) { >> + /* >> + * Write on bootsector. Allow only changing the reserved1 field, >> + * used to mark volume dirtiness >> + */ >> + const unsigned char *initial = s->first_sectors >> + + s->offset_to_bootsector * 0x200; >> + for (i = 0; i < 0x200; i++) { >> + if (i != offsetof(bootsector_t, u.fat16.reserved1) && > > I think you need to check the FAT version (s->fat_type) before accessing > u.fat16. For FAT32, the "reserved" field is at a different offset (but > seems to have the same meaning). I didn't do this, because only fat16 part of bootsector is ever used. In init_directories(), only fat16 part is initialized, with the comment: /* LATER TODO: if FAT32, this is wrong */ I wanted to be consistent between init_directories() and the check. > >> + initial[i] != buf[i]) { >> + fprintf(stderr, "Tried to write to protected bootsector\n"); >> + return -1; >> + } >> + } >> + return 0; >> + } > > Should we update s->first_sectors with the new value so that the guest > would actually read back what it wrote instead of having the change > disappear magically? Windows guests don't seem to care if the written value disappears. They only want the write to succeed. > >> /* >> * Some sanity checks: >> * - do not allow writing to the boot sector >> */ >> - >> if (sector_num < s->offset_to_fat) >> return -1; > > Kevin >
Am 29.09.2022 um 21:53 hat Hervé Poussineau geschrieben: > Le 29/09/2022 à 16:10, Kevin Wolf a écrit : > > Am 03.09.2022 um 18:23 hat Hervé Poussineau geschrieben: > > > 'reserved1' field in bootsector is used to mark volume dirty, or need to verify. > > > Allow writes to bootsector which only changes the 'reserved1' field. > > > > > > This fixes I/O errors on Windows guests. > > > > > > Resolves: https://bugs.launchpad.net/qemu/+bug/1889421 > > > Signed-off-by: Hervé Poussineau <hpoussin@reactos.org> > > > --- > > > block/vvfat.c | 18 +++++++++++++++++- > > > 1 file changed, 17 insertions(+), 1 deletion(-) > > > > > > diff --git a/block/vvfat.c b/block/vvfat.c > > > index d6dd919683d..35057a51c67 100644 > > > --- a/block/vvfat.c > > > +++ b/block/vvfat.c > > > @@ -2993,11 +2993,27 @@ DLOG(checkpoint()); > > > vvfat_close_current_file(s); > > > + if (sector_num == s->offset_to_bootsector && nb_sectors == 1) { > > > + /* > > > + * Write on bootsector. Allow only changing the reserved1 field, > > > + * used to mark volume dirtiness > > > + */ > > > + const unsigned char *initial = s->first_sectors > > > + + s->offset_to_bootsector * 0x200; > > > + for (i = 0; i < 0x200; i++) { > > > + if (i != offsetof(bootsector_t, u.fat16.reserved1) && > > > > I think you need to check the FAT version (s->fat_type) before accessing > > u.fat16. For FAT32, the "reserved" field is at a different offset (but > > seems to have the same meaning). > > I didn't do this, because only fat16 part of bootsector is ever used. > In init_directories(), only fat16 part is initialized, with the comment: > /* LATER TODO: if FAT32, this is wrong */ > I wanted to be consistent between init_directories() and the check. Oh, indeed. I guess this means FAT32 is completely broken... Fair enough, though maybe we could add a similar comment here, then. > > > + initial[i] != buf[i]) { > > > + fprintf(stderr, "Tried to write to protected bootsector\n"); > > > + return -1; > > > + } > > > + } > > > + return 0; > > > + } > > > > Should we update s->first_sectors with the new value so that the guest > > would actually read back what it wrote instead of having the change > > disappear magically? > > Windows guests don't seem to care if the written value disappears. > They only want the write to succeed. But it would be arguably more correct, wouldn't it? Some other OS might care. Kevin
diff --git a/block/vvfat.c b/block/vvfat.c index d6dd919683d..35057a51c67 100644 --- a/block/vvfat.c +++ b/block/vvfat.c @@ -2993,11 +2993,27 @@ DLOG(checkpoint()); vvfat_close_current_file(s); + if (sector_num == s->offset_to_bootsector && nb_sectors == 1) { + /* + * Write on bootsector. Allow only changing the reserved1 field, + * used to mark volume dirtiness + */ + const unsigned char *initial = s->first_sectors + + s->offset_to_bootsector * 0x200; + for (i = 0; i < 0x200; i++) { + if (i != offsetof(bootsector_t, u.fat16.reserved1) && + initial[i] != buf[i]) { + fprintf(stderr, "Tried to write to protected bootsector\n"); + return -1; + } + } + return 0; + } + /* * Some sanity checks: * - do not allow writing to the boot sector */ - if (sector_num < s->offset_to_fat) return -1;
'reserved1' field in bootsector is used to mark volume dirty, or need to verify. Allow writes to bootsector which only changes the 'reserved1' field. This fixes I/O errors on Windows guests. Resolves: https://bugs.launchpad.net/qemu/+bug/1889421 Signed-off-by: Hervé Poussineau <hpoussin@reactos.org> --- block/vvfat.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)