19.11.2019 15:20, Max Reitz wrote: > On 19.11.19 13:02, Denis V. Lunev wrote: >> On 11/19/19 1:22 PM, Max Reitz wrote: >>> On 16.11.19 17:34, Vladimir Sementsov-Ogievskiy wrote: >>>> Hi all! >>>> >>>> I wanted to understand, what is the real difference between >>>> bdrv_block_status_above >>>> and bdrv_is_allocated_above, IMHO bdrv_is_allocated_above should work >>>> through >>>> bdrv_block_status_above.. >>>> >>>> And I found the problem: bdrv_is_allocated_above considers space after EOF >>>> as >>>> UNALLOCATED for intermediate nodes.. >>>> >>>> UNALLOCATED is not about allocation at fs level, but about should we go to >>>> backing or >>>> not.. And it seems incorrect for me, as in case of short backing file, >>>> we'll read >>>> zeroes after EOF, instead of going further by backing chain. >>> Should we, though? It absolutely makes sense to me to consider post-EOF >>> space as unallocated because, well, it is as unallocated as it gets. >>> >>> So from my POV it would make more sense to fall back to the backing file >>> for post-EOF reads. >>> >>> OTOH, I don’t know whether changing that behavior would qualify as a >>> possible security issue now, because maybe someone has sensitive >>> information in the tail of some disk and then truncated the overlay so >>> as to hide it? But honestly, that seems ridiculous and I can’t imagine >>> people to do that. (It would work only for the tail, and why not just >>> write zeroes there, which works everywhere?) So in practice I don’t >>> believe that to be a problem. >>> >>> Max >> >> That seems to be wrong from my POW. Once we get block device truncated, >> it exposed that tail to the guest with all zeroes. >> >> Let us assume that we have virtual disk of length L. We create new top >> delta of >> length X (less then L) and new top delta after with length Y (more than L), >> like the following: >> >> [.........................] Y >> [........] X >> [...................] L >> >> Once the guest creates FS on state Y it relies on the fact that data from X >> to Y is all zeroes. >> >> Any operations with backing chain must keep guest content to be tha same, >> i.e. if we commit from Y to L, virtual disk content should be preserved, >> i.e. >> read as all zero even if there is some data in L from X to L. >> >> If we commit from X to Y, the range from X to L should remain all zeroes. >> >> This is especially valid for backups, which can not be changed and are >> validated by the software from time to time. >> >> Does this makes sense? > > All right then. But then there’s the case of commit not shrinking the > backing file, so the guest content won’t be the same if you commit a > short overlay into a longer backing file. >
Hmm. Isn't commit target truncated to source before operation? -- Best regards, Vladimir
