Date: Mon, 10 Jun 2019 07:37:11 -0700 From: Conrad Meyer <cem@freebsd.org> To: Mark Millard <marklmi@yahoo.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org>, freeBSD PowerPC ML <freebsd-ppc@freebsd.org> Subject: Re: kern_execve using vm_page_zero_invalid but not vm_page_set_validclean to load /sbin/init ? Message-ID: <CAG6CVpV5FBHgOTgxEgRmP%2B46Vm7mxoPCPECDJiq3k=D4qZ8PCA@mail.gmail.com> In-Reply-To: <1464D960-A1D6-404A-BB10-E615E2D14C1D@yahoo.com> References: <1464D960-A1D6-404A-BB10-E615E2D14C1D@yahoo.com>
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Hi Mark, On Sun, Jun 9, 2019 at 11:17 PM Mark Millard via freebsd-hackers <freebsd-hackers@freebsd.org> wrote: > ... > vm_pager_get_pages uses vm_page_zero_invalid > to "Zero out partially filled data". > > But vm_page_zero_invalid does not zero every "invalid" > byte but works in terms of units of DEV_BSIZE : > ... > The comment indicates that areas of "sub-DEV_BSIZE" > should have been handled previously by > vm_page_set_validclean . Or another VM routine, yes (e.g., vm_page_set_valid_range). The valid and dirty bitmasks in vm_page only have a single bit per DEV_BSIZE region, so care must be taken when marking any sub-DEV_BSIZE region as valid to zero out the rest of the DEV_BSIZE region. This is part of the VM page contract. I'm not sure it's related to the BSS, though. > So, if, say, char**environ ends up at the start of .sbss > consistently, does environ always end up zeroed independently > of FileSz for the PT_LOAD that spans them? It is required to be zeroed, yes. If not, there is a bug. If FileSz covers BSS, that's a bug in the linker. Either the trailing bytes of the corresponding page in the executable should be zero (wasteful; on amd64 ".comment" is packed in there instead), or the linker/loader must zero them at initialization. I'm not familiar with the particular details here, but if you are interested I would suggest looking at __elfN(load_section) in sys/kern/imgact_elf.c. > The following is not necessarily an example of problematical > figures but is just for showing an example structure of what > FileSiz covers vs. MemSiz for PT_LOAD's that involve .sbss > and .bss : > ... Your 2nd LOAD phdr's FileSiz matches up exactly with Segment .sbss Offset minus Segment .tdata Offset, i.e., none of the FileSiz corresponds to the (s)bss regions. (Good! At least the static linker part looks sane.) That said, the boundary is not page-aligned and the section alignment requirement is much lower than page_size, so the beginning of bss will share a file page with some data. Something should zero it at image activation. (Tangent: sbss/bss probably do not need to be RWE on PPC! On amd64, init has three LOAD segments rather than two: one for rodata (R), one for .text, .init, etc (RX); and one for .data (RW).) Best, Conrad
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