D31317.diff
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	diff --git a/share/man/man4/pci.4 b/share/man/man4/pci.4
	--- a/share/man/man4/pci.4
	+++ b/share/man/man4/pci.4
	@@ -24,12 +24,12 @@
	.\"
	.\" $FreeBSD$
	.\"
	-.Dd June 17, 2019
	+.Dd July 27, 2021
	.Dt PCI 4
	.Os
	.Sh NAME
	.Nm pci
	-.Nd generic PCI bus driver
	+.Nd generic PCI/PCIe bus driver
	.Sh SYNOPSIS
	To compile the PCI bus driver into the kernel,
	place the following line in your
	@@ -53,6 +53,8 @@
	.Nm
	driver provides support for
	.Tn PCI
	+and
	+.Tn PCIe
	devices in the kernel and limited access to
	.Tn PCI
	devices for userland.
	@@ -80,6 +82,15 @@
	device.
	If used improperly, this driver can allow userland applications to
	crash a machine or cause data loss.
	+In particular, driver only allows operations on the opened
	+.Pa /dev/pci
	+to modify system state if the file descriptor was opened for writing.
	+For instance, the
	+.Dv PCIOCREAD
	+and
	+.Dv PCIOCBARMMAP
	+operations require a writeable descriptor, because reading a config register
	+or a BAR read access could have function-specific side-effects.
	.Pp
	The
	.Nm
	diff --git a/sys/dev/pci/pci_user.c b/sys/dev/pci/pci_user.c
	--- a/sys/dev/pci/pci_user.c
	+++ b/sys/dev/pci/pci_user.c
	@@ -947,7 +947,12 @@
	io_old = NULL;
	#endif

	- if (!(flag & FWRITE)) {
	+ /*
	+ * Interpret read-only opened /dev/pci as a promise that no
	+ * operation of the file descriptor could modify system state,
	+ * including side-effects due to reading devices registers.
	+ */
	+ if ((flag & FWRITE) == 0) {
	switch (cmd) {
	case PCIOCGETCONF:
	#ifdef COMPAT_FREEBSD32