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PCI/sysfs: Avoid spurious runtime PM wakeup on config space accesses

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Currently, the boundary checks in pci_read_config() and pci_write_config()
reject only offsets beyond the effective configuration space size.

An access at an offset exactly equal to that size passes the check, has its
length clamped to zero, and then invokes pci_config_pm_runtime_get() and
pci_config_pm_runtime_put() around transfer blocks that do nothing.

This is a problem because pci_config_pm_runtime_get() synchronously resumes
the upstream bridge through pm_runtime_get_sync() and resumes the device
itself through pm_runtime_resume() when it is in D3cold, only for the
handler to return zero immediately afterwards.  Such a spurious wakeup
wastes power and adds needless resume latency.

The sysfs core already clamps accesses against the attribute size set
through the bin_size() callback, which reports either 256 or 4096 bytes.
As such, the affected accesses are reads at offset 64 (or 128 for CardBus
devices) through files opened without CAP_SYS_ADMIN, and reads and writes
at the exact configuration space size on devices where a quirk sets a
non-standard size.

Reject accesses at the boundary offset as well, so they return early before
any runtime PM involvement, matching the procfs implementations in
proc_bus_pci_read() and proc_bus_pci_write().

The value returned to userspace at these offsets remains zero, so the
change is not visible to userspace.

Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
[bhelgaas: tweak commit log, order tags]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260720204356.1501749-1-kwilczynski@kernel.org
This commit is contained in:
Krzysztof Wilczyński 2026-07-20 20:43:56 +00:00 committed by Bjorn Helgaas
parent 651fb94aaf
commit b14b2bab88
1 changed files with 2 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
drivers/pci/pci-sysfs.c
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@ -718,7 +718,7 @@ static ssize_t pci_read_config(struct file *filp, struct kobject *kobj,
else if (dev->hdr_type == PCI_HEADER_TYPE_CARDBUS)
size = 128;
if (off > size)
if (off >= size)
return 0;
if (off + count > size) {
size -= off;
@ -799,7 +799,7 @@ static ssize_t pci_write_config(struct file *filp, struct kobject *kobj,
add_taint(TAINT_USER, LOCKDEP_STILL_OK);
}
if (off > dev->cfg_size)
if (off >= dev->cfg_size)
return 0;
if (off + count > dev->cfg_size) {
size = dev->cfg_size - off;