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Re: [RFC PATCH 3/4] xen/arm: initialize conditionally uninitialized local variables
Hi,
On 19/07/2023 14:27, Nicola Vetrini wrote:
On 14/07/23 15:21, Julien Grall wrote:
Hi,
On 14/07/2023 12:49, Nicola Vetrini wrote:
This patch aims to fix some occurrences of possibly uninitialized
variables, that may be read before being written. This behaviour would
violate MISRA C:2012 Rule 9.1, besides being generally undesirable.
In all the analyzed cases, such accesses were actually safe, but it's
quite difficult to prove so by automatic checking, therefore a safer
route is to change the code so as to avoid the behaviour from occurring,
while preserving the semantics.
An initialization to a safe value is provided to reach this aim.
Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
Additional input on which values may be 'safe' in each context is
surely welcome, to avoid possibly compromising the correctness of
the function semantics.
---
xen/arch/arm/cpuerrata.c | 6 +++---
xen/arch/arm/domctl.c | 8 ++++----
xen/arch/arm/gic-v3-lpi.c | 17 +++++++++--------
xen/arch/arm/include/asm/p2m.h | 10 ++++++----
xen/arch/arm/platforms/xilinx-zynqmp-eemi.c | 10 ++--------
xen/arch/arm/psci.c | 10 +++++-----
xen/drivers/char/pl011.c | 2 +-
7 files changed, 30 insertions(+), 33 deletions(-)
diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
index d0658aedb6..14694c6081 100644
--- a/xen/arch/arm/cpuerrata.c
+++ b/xen/arch/arm/cpuerrata.c
@@ -159,7 +159,7 @@ extern char __mitigate_spectre_bhb_loop_start_32[],
static int enable_smccc_arch_workaround_1(void *data)
{
- struct arm_smccc_res res;
+ struct arm_smccc_res res = {0};
I understand you desire to make happy. But I am not sure that
initializing to 0 is the right thing. If the SMCC were not properly
setting the register, then we most likely don't want to install the
workaround. Instead, we most likely want to warn.
So you want (int)res.a0 to be negative. We don't care about the other
fields.
In principle I'm ok with this, but see below.
const struct arm_cpu_capabilities *entry = data;
/*
@@ -252,7 +252,7 @@ static int enable_spectre_bhb_workaround(void *data)
if ( cpus_have_cap(ARM_WORKAROUND_BHB_SMCC_3) )
{
- struct arm_smccc_res res;
+ struct arm_smccc_res res = {0};
Same remark here.
if ( smccc_ver < SMCCC_VERSION(1, 1) )
goto warn;
@@ -393,7 +393,7 @@ DEFINE_PER_CPU_READ_MOSTLY(register_t,
ssbd_callback_required);
static bool has_ssbd_mitigation(const struct arm_cpu_capabilities
*entry)
{
- struct arm_smccc_res res;
+ struct arm_smccc_res res = {0};
Here you would want (int)res.a0 to be equal to ARM_SMCCC_NOT_SUPPORTED.
I see that ARM_SMCCC_NOT_SUPPORTED is
#define ARM_SMCCC_NOT_SUPPORTED (-1)
thus an assignment to res.a0 would violate Rule 10.3:
"The value of an expression shall not be assigned to an object with a
narrower essential type or of a different essential type category."
(signed vs unsigned, and the exception does not apply here).
This rule is not yet under discussion, but I would like to avoid
knowingly introducing more violations if there's an alternative.
Do the fields of struct arm_smccc_res really need to be unsigned?
Yes, all the fields represent a register. Also, in this context, only
the first 32-bit of the register should be taken into account.
That why you will see code using (int)res.a0.
bool required;
if ( smccc_ver < SMCCC_VERSION(1, 1) )
diff --git a/xen/arch/arm/domctl.c b/xen/arch/arm/domctl.c
index ad56efb0f5..b38fed72be 100644
--- a/xen/arch/arm/domctl.c
+++ b/xen/arch/arm/domctl.c
@@ -29,10 +29,10 @@ static int handle_vuart_init(struct domain *d,
struct xen_domctl_vuart_op *vuart_op)
{
int rc;
- struct vpl011_init_info info;
-
- info.console_domid = vuart_op->console_domid;
- info.gfn = _gfn(vuart_op->gfn);
+ struct vpl011_init_info info = {
+ .console_domid = vuart_op->console_domid,
+ .gfn = _gfn(vuart_op->gfn)
+ };
I am not against, this change. But I don't quite understand how this
makes Eclair much happier?
It also zero-initializes the third field:
struct vpl011_init_info {
domid_t console_domid;
gfn_t gfn;
evtchn_port_t evtchn;
};
Also, if this is the desired way, then I think this should be written
down in the CODING_STYLE.
This is just a matter of style, I can also set the other field
explicitly, if you prefer.
I am confused. In a previous reply, I thought you said the following
would also make ECLAIR unhappy:
info.console_domid = <...>;
info.gfn = <...>;
info.evtchn = <...>;
Either way, it might be a good idea to settle
this in the CODING_STYLE in a different patch.
Well... The pattern you complain about is fairly common in Xen and I
personally prefer to use in some situation over your proposal. So I
think we need to agree first that your proposal is the way to go before
modifying the code everywhere.
[...]
I understand the exist construct is not liked by Eclair. But your new
construct is not easily readable by a human. Before I can propose a
different approach, can you clarify why Eclair is unhappy this the
existing code?
This has been addressed in the reply to patch 1/4.
AFAICT the conversation wasn't settled. We will continue there to avoid
duplication.
/*
* DOMID_XEN sees 1-1 RAM. The p2m_type is based on the type of
the
diff --git a/xen/arch/arm/platforms/xilinx-zynqmp-eemi.c
b/xen/arch/arm/platforms/xilinx-zynqmp-eemi.c
index 2053ed7ac5..39d9ab4fa9 100644
--- a/xen/arch/arm/platforms/xilinx-zynqmp-eemi.c
+++ b/xen/arch/arm/platforms/xilinx-zynqmp-eemi.c
@@ -51,11 +51,11 @@ static inline bool domain_has_reset_access(struct
domain *d, uint32_t rst)
bool zynqmp_eemi(struct cpu_user_regs *regs)
{
- struct arm_smccc_res res;
+ struct arm_smccc_res res = {0};
uint32_t fid = get_user_reg(regs, 0);
uint32_t nodeid = get_user_reg(regs, 1);
unsigned int pm_fn = fid & 0xFFFF;
- enum pm_ret_status ret;
+ enum pm_ret_status ret = XST_PM_NO_ACCESS;
It is not clear to me why Eclair is unhappy here?
Gotos, as explained in the reply to patch 1/4. Since 'ret' here already
sets a value in every branch, I am in favour of initializing it before
the switch with that value. The zero-ed 'arm_smccc_res' is ok here?
I am not in favor of this approach. Have you considered my proposal...
switch ( fid )
{
@@ -89,7 +89,6 @@ bool zynqmp_eemi(struct cpu_user_regs *regs)
{
gprintk(XENLOG_WARNING,
"zynqmp-pm: fn=%u No access to node %u\n",
pm_fn, nodeid);
- ret = XST_PM_NO_ACCESS;
goto done;
I find the new approach less obvious. How about introduce a macro that
will set the result and the use replace the two lines with:
set_result(regs, XST_PM_NO_ACCESS);
return 0;
where set_result(regs, ret) is
set_user_reg(regs, 0, ret)
... here?
[...]
diff --git a/xen/drivers/char/pl011.c b/xen/drivers/char/pl011.c
index f7bf3ad117..34ce90be52 100644
--- a/xen/drivers/char/pl011.c
+++ b/xen/drivers/char/pl011.c
@@ -285,7 +285,7 @@ static int __init pl011_dt_uart_init(struct
dt_device_node *dev,
const char *config = data;
int res;
paddr_t addr, size;
- uint32_t io_width;
+ uint32_t io_width = 0;
We have many use of dt_property_read_*() within the code base. I would
like us te decide of a policy first.
Well, the best outcome for me is if all these functions (i.e.,
dt_property_*, dt_get_property_*) always write the parameter they are
supposed to set. I understand if this is not always feasible, but the
alternative is not clean (e.g., deviate or refactor in such a way that
the code is less readable).
I have had a look at the helpers. Here some thoughts
* dt_get_property(): Always setting '*lenp' could result to user
starting relying on the 'len' rather than the error code. This could
potentially be mitigated by forcing the code to check the value (i.e.
adding __must_check).
* dt_property_read_u*(): The best would be to let the user decide what
would a default sane value by adding an extra parameter.
* dt_property_read_string(): I would return the string and use
ERR_PTR(...) when there is an error.
Cheers,
--
Julien Grall
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