Alex, thank you for the review. On Fri, Mar 05, 2021 at 02:03:43PM +0000, Alex Bennée wrote: > > Ivan A. Melnikov <i...@altlinux.org> writes: > > > While pgd_find_hole_fallback returns the beginning of the > > hole found, pgb_find_hole returns guest_base, which > > is somewhat different as the binary qemu-user is loading > > usually has non-zero load address. > > > > Failing to take this into account leads to random crashes > > if the hole is "just big enough", but not bigger: > > in that case, important mappings (e.g. parts of qemu-user > > itself) may be replaced with the binary it is loading > > (e.g. the guest elf interpreter). > > > > This patch also fixes the return type of pgd_find_hole_fallback: > > it returns -1 if no hole is found, so a signed return type > > should be used. > > I don't think it should. For one thing the type is preserved as > uintptr_t all the way up the call chain so just changing it here doesn't > help much. -1 is really just a quick way of saying all bits are set > which is the one "fail" value we check for. The address space is big > enough we could theoretically return a chunk of space that otherwise has > the top bit set.
I see your point. I'd only suggest to be explicit about using all-ones as a spectial value, something like this: static const uintptr_t invalid_poitner = ~0ULL; and then using it as a return value. Especially since, as far as I remeber, comparing unsigned value with -1 (which is int) is UB. Makes sense? > > Downstream issue (in Russian): https://bugzilla.altlinux.org/39141 > > Signed-off-by: Ivan A. Melnikov <i...@altlinux.org> > > --- > > linux-user/elfload.c | 11 +++++++---- > > 1 file changed, 7 insertions(+), 4 deletions(-) > > > > diff --git a/linux-user/elfload.c b/linux-user/elfload.c > > index bab4237e90..acd510532c 100644 > > --- a/linux-user/elfload.c > > +++ b/linux-user/elfload.c > > @@ -2205,9 +2205,11 @@ static void pgb_have_guest_base(const char > > *image_name, abi_ulong guest_loaddr, > > * /proc/self/map. It can potentially take a very long time as we can > > * only dumbly iterate up the host address space seeing if the > > * allocation would work. > > + * > > + * Returns the start addres of the hole found, or -1 if no hole found. > > */ > > -static uintptr_t pgd_find_hole_fallback(uintptr_t guest_size, uintptr_t > > brk, > > - long align, uintptr_t offset) > > +static intptr_t pgd_find_hole_fallback(uintptr_t guest_size, uintptr_t brk, > > + long align, uintptr_t offset) > > { > > uintptr_t base; > > > > @@ -2235,7 +2237,7 @@ static uintptr_t pgd_find_hole_fallback(uintptr_t > > guest_size, uintptr_t brk, > > munmap((void *) align_start, guest_size); > > if (MAP_FIXED_NOREPLACE != 0 || > > mmap_start == (void *) align_start) { > > - return (uintptr_t) mmap_start + offset; > > + return (intptr_t) mmap_start + offset; > > } > > } > > base += qemu_host_page_size; > > @@ -2259,7 +2261,8 @@ static uintptr_t pgb_find_hole(uintptr_t > > guest_loaddr, uintptr_t guest_size, > > brk = (uintptr_t)sbrk(0); > > > > if (!maps) { > > - return pgd_find_hole_fallback(guest_size, brk, align, offset); > > + ret = pgd_find_hole_fallback(guest_size, brk, align, offset); > > + return (ret > guest_loaddr) ? (ret - guest_loaddr) : -1; > > So I think we just want: > > return ret == -1 ? -1 : (ret - guest_loaddr); This will work for me as well. I'm just a bit hesitant with putting in some kind of integer underflow when it seems easily avoidable; but if we stick with uintptr_t that should non matter. > do we have a test case that triggers this? I don't think there are test cases that cover the pgd_find_hole_fallback code path, at least when the test suite is run on a system with /proc mounted. -- wbr, iv m.
