Messages in this thread | | | Date | Mon, 15 Oct 2018 08:44:04 -0500 | From | Bjorn Helgaas <> | Subject | Re: [PATCH 1/3] x86/kexec: Correct KEXEC_BACKUP_SRC_END off-by-one error |
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On Mon, Oct 15, 2018 at 12:51:38PM +0800, Dave Young wrote: > On 09/30/18 at 05:27pm, Dave Young wrote: > > On 09/30/18 at 05:21pm, Dave Young wrote: > > > On 09/27/18 at 09:21am, Bjorn Helgaas wrote: > > > > From: Bjorn Helgaas <bhelgaas@google.com> > > > > > > > > The only use of KEXEC_BACKUP_SRC_END is as an argument to > > > > walk_system_ram_res(): > > > > > > > > int crash_load_segments(struct kimage *image) > > > > { > > > > ... > > > > walk_system_ram_res(KEXEC_BACKUP_SRC_START, KEXEC_BACKUP_SRC_END, > > > > image, determine_backup_region); > > > > > > > > walk_system_ram_res() expects "start, end" arguments that are inclusive, > > > > i.e., the range to be walked includes both the start and end addresses. > > > > > > Looking at the function comment of find_next_iomem_res, the res->end > > > should be exclusive, am I missing something? > > > > Oops, you fix it in 2nd patch, I apparently miss that. > > > > Since the fix of checking the end is in another patch, probably merge > > these two patches so that they are in one patch to avoid break bisect. > > Not sure if above comment was missed, Boris, would you mind to fold the > patch 1 and 2?
Sorry, I did miss this comment.
Patch 2 was for the very specific case of a single-byte resource at the end address, which we probably never see in practice.
For patch 1, the find_next_iomem_res() function comment had "[res->start.res->end)", but I think the code actually treated it as "[res->start.res->end]", so the comment was inaccurate.
Before my patches we had:
#define KEXEC_BACKUP_SRC_START (0UL) #define KEXEC_BACKUP_SRC_END (640 * 1024UL) # 0xa0000
The intention is to search for system RAM resources that intersect this region:
[mem 0x0-0x9ffff]
The call is:
walk_system_ram_res(KEXEC_BACKUP_SRC_START, KEXEC_BACKUP_SRC_END, ..., determine_backup_region); walk_system_ram_res(0, 0xa0000, ..., determine_backup_region);
Assume iomem_resource contains this system RAM resource:
[mem 0x90000-0xaffff]
In find_next_iomem_res(), the "res" input parameter is the region to search:
res->start = 0; # KEXEC_BACKUP_SRC_START res->end = 0xa0000; # KEXEC_BACKUP_SRC_END
In one of the loop iterations we find the [mem 0x90000-0xaffff] resource (p):
p->start = 0x90000; p->end = 0xaffff;
if (p->start > end) # 0x90000 > 0xa0000? false if (p->end >= start && p->start < end) # 0xaffff >= 0 ? true # 0x90000 < 0xa0000 ? true break; # so we'll return part of "p"
if (res->start < p->start) # 0x0 < 0x90000 ? true res->start = 0x90000; # trim beginning to p->start if (res->end > p->end) # 0xa0000 > 0xaffff ? false
So find_next_iomem_res() returns with this:
res->start = 0x90000; # trimmed to p->start res->end = 0xa0000; # unchanged from input
[mem 0x90000-0xa0000] # returned resource (res)
and we call determine_backup_region(res), which sets:
image->arch.backup_src_start = 0x90000; image->arch.backup_src_sz = resource_size(res) # 0xa0000 - 0x90000 + 1 # (0x10001)
This is incorrect. What we wanted was the part of [mem 0x90000-0xaffff] that intersects the first 640K, i.e., [mem 0x90000-0x9ffff], but what we got was [mem 0x90000-0xa0000], which is one byte too long.
The resource returned find_next_iomem_res() always ends at the "res->end" supplied as an input parameter *unless* the input res->end is strictly greater than the p->end, when it is truncated to p->end.
Bottom line, I don't think patches 1 and 2 need to be folded together because they fix different problems.
Bjorn
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