[PATCH v2 0/1] plugins: fix syscall filter return value type

[PATCH v2 0/1] plugins: fix syscall filter return value type

[Ziyang Zhang]

Hi,

The sysret value of the syscall filter callback is declared as uint64_t,
but it is semantically signed: the negative range encodes errno. This
makes it int64_t, matching the ret parameter of
qemu_plugin_vcpu_syscall_ret_cb_t, which already uses int64_t for the
same syscall return value.

As discussed on v1, this is not a correctness fix (the filter only sets
sysret, so the bit pattern is the same either way); it is a consistency
and readability improvement, so that the same syscall return value is
typed the same way on the read side and the write side.

I did not bump QEMU_PLUGIN_VERSION, since this only changes the
signedness of an existing argument.

v1: https://lore.kernel.org/qemu-devel/20260612054800.587419-1-functioner@sjtu.edu.cn/

Changes since v1:
- rebased onto current master, where the syscall callbacks now take a
  userdata argument; the filter signatures were adjusted to keep that
  while changing only the sysret type. No functional change otherwise.

Thanks for your review.

Ziyang Zhang (1):
  plugins: use int64_t for the syscall filter return value

 include/plugins/qemu-plugin.h | 2 +-
 include/qemu/plugin.h         | 4 ++--
 linux-user/syscall.c          | 2 +-
 plugins/core.c                | 2 +-
 tests/tcg/plugins/setpc.c     | 2 +-
 tests/tcg/plugins/syscall.c   | 2 +-
 6 files changed, 7 insertions(+), 7 deletions(-)

-- 
2.34.1

[PATCH v2 1/1] plugins: use int64_t for the syscall filter return value

[Ziyang Zhang]

The syscall return value passed back through the syscall filter
callback is semantically signed: negative values encode errno codes.
Declaring the sysret pointer as uint64_t * is therefore misleading and
forces callers to launder the value through an unsigned temporary.

Change the sysret pointer to int64_t * across the public plugin API
typedef (qemu_plugin_vcpu_syscall_filter_cb_t), the internal
qemu_plugin_vcpu_syscall_filter() prototypes and stub, its
implementation in plugins/core.c, the linux-user caller, and the
in-tree example plugins.

Signed-off-by: Ziyang Zhang <functioner@sjtu.edu.cn>
---
v2: rebased onto current master. The syscall callbacks now take a
    userdata argument, so the filter signatures were adjusted to keep
    that while changing only the sysret type. No functional change.
---
 include/plugins/qemu-plugin.h | 2 +-
 include/qemu/plugin.h         | 4 ++--
 linux-user/syscall.c          | 2 +-
 plugins/core.c                | 2 +-
 tests/tcg/plugins/setpc.c     | 2 +-
 tests/tcg/plugins/syscall.c   | 2 +-
 6 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/plugins/qemu-plugin.h b/include/plugins/qemu-plugin.h
index d8d4b37384..3e913cd89c 100644
--- a/include/plugins/qemu-plugin.h
+++ b/include/plugins/qemu-plugin.h
@@ -871,7 +871,7 @@ typedef bool
                                         int64_t num, uint64_t a1, uint64_t a2,
                                         uint64_t a3, uint64_t a4, uint64_t a5,
                                         uint64_t a6, uint64_t a7, uint64_t a8,
-                                        uint64_t *sysret,
+                                        int64_t *sysret,
                                         void *userdata);
 
 /**
diff --git a/include/qemu/plugin.h b/include/qemu/plugin.h
index 9356ee836a..9c2ec8ceae 100644
--- a/include/qemu/plugin.h
+++ b/include/qemu/plugin.h
@@ -172,7 +172,7 @@ bool
 qemu_plugin_vcpu_syscall_filter(CPUState *cpu, int64_t num, uint64_t a1,
                                 uint64_t a2, uint64_t a3, uint64_t a4,
                                 uint64_t a5, uint64_t a6, uint64_t a7,
-                                uint64_t a8, uint64_t *sysret);
+                                uint64_t a8, int64_t *sysret);
 
 void qemu_plugin_vcpu_mem_cb(CPUState *cpu, uint64_t vaddr,
                              uint64_t value_low,
@@ -288,7 +288,7 @@ static inline bool
 qemu_plugin_vcpu_syscall_filter(CPUState *cpu, int64_t num, uint64_t a1,
                                 uint64_t a2, uint64_t a3, uint64_t a4,
                                 uint64_t a5, uint64_t a6, uint64_t a7,
-                                uint64_t a8, uint64_t *sysret)
+                                uint64_t a8, int64_t *sysret)
 {
     return false;
 }
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index fbe357b7e0..d257fb9ca9 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -14619,7 +14619,7 @@ static bool send_through_syscall_filters(CPUState *cpu, int num,
                                          abi_long arg7, abi_long arg8,
                                          abi_long *sysret)
 {
-    uint64_t sysret64 = 0;
+    int64_t sysret64 = 0;
     bool filtered = qemu_plugin_vcpu_syscall_filter(cpu, num, arg1, arg2,
                                                     arg3, arg4, arg5, arg6,
                                                     arg7, arg8, &sysret64);
diff --git a/plugins/core.c b/plugins/core.c
index 4b55aacd2d..1113bfe567 100644
--- a/plugins/core.c
+++ b/plugins/core.c
@@ -573,7 +573,7 @@ bool
 qemu_plugin_vcpu_syscall_filter(CPUState *cpu, int64_t num, uint64_t a1,
                                 uint64_t a2, uint64_t a3, uint64_t a4,
                                 uint64_t a5, uint64_t a6, uint64_t a7,
-                                uint64_t a8, uint64_t *sysret)
+                                uint64_t a8, int64_t *sysret)
 {
     struct qemu_plugin_cb *cb, *next;
     enum qemu_plugin_event ev = QEMU_PLUGIN_EV_VCPU_SYSCALL_FILTER;
diff --git a/tests/tcg/plugins/setpc.c b/tests/tcg/plugins/setpc.c
index 7c78f182f0..76b2efc61b 100644
--- a/tests/tcg/plugins/setpc.c
+++ b/tests/tcg/plugins/setpc.c
@@ -27,7 +27,7 @@ static bool vcpu_syscall_filter(unsigned int vcpu_index,
                                 int64_t num, uint64_t a1, uint64_t a2,
                                 uint64_t a3, uint64_t a4, uint64_t a5,
                                 uint64_t a6, uint64_t a7, uint64_t a8,
-                                uint64_t *sysret, void *userdata)
+                                int64_t *sysret, void *userdata)
 {
     if (num == MAGIC_SYSCALL) {
         if (a1 == SETPC) {
diff --git a/tests/tcg/plugins/syscall.c b/tests/tcg/plugins/syscall.c
index a28d108784..debec9f09b 100644
--- a/tests/tcg/plugins/syscall.c
+++ b/tests/tcg/plugins/syscall.c
@@ -176,7 +176,7 @@ static bool vcpu_syscall_filter(unsigned int vcpu_index,
                                 int64_t num, uint64_t a1, uint64_t a2,
                                 uint64_t a3, uint64_t a4, uint64_t a5,
                                 uint64_t a6, uint64_t a7, uint64_t a8,
-                                uint64_t *sysret, void *userdata)
+                                int64_t *sysret, void *userdata)
 {
     /* Special syscall to test the filter functionality. */
     if (num == 4096 && a1 == 0x66CCFF) {
-- 
2.34.1

Re: [PATCH v2 1/1] plugins: use int64_t for the syscall filter return value

[Pierrick Bouvier]

On 6/18/2026 1:24 AM, Ziyang Zhang wrote:
> The syscall return value passed back through the syscall filter
> callback is semantically signed: negative values encode errno codes.
> Declaring the sysret pointer as uint64_t * is therefore misleading and
> forces callers to launder the value through an unsigned temporary.
> 
> Change the sysret pointer to int64_t * across the public plugin API
> typedef (qemu_plugin_vcpu_syscall_filter_cb_t), the internal
> qemu_plugin_vcpu_syscall_filter() prototypes and stub, its
> implementation in plugins/core.c, the linux-user caller, and the
> in-tree example plugins.
> 
> Signed-off-by: Ziyang Zhang <functioner@sjtu.edu.cn>
> ---
> v2: rebased onto current master. The syscall callbacks now take a
>     userdata argument, so the filter signatures were adjusted to keep
>     that while changing only the sysret type. No functional change.
> ---
>  include/plugins/qemu-plugin.h | 2 +-
>  include/qemu/plugin.h         | 4 ++--
>  linux-user/syscall.c          | 2 +-
>  plugins/core.c                | 2 +-
>  tests/tcg/plugins/setpc.c     | 2 +-
>  tests/tcg/plugins/syscall.c   | 2 +-
>  6 files changed, 7 insertions(+), 7 deletions(-)
> 

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>

Re: [PATCH v2 1/1] plugins: use int64_t for the syscall filter return value

[Alex Bennée]

Ziyang Zhang <functioner@sjtu.edu.cn> writes:

> The syscall return value passed back through the syscall filter
> callback is semantically signed: negative values encode errno codes.
> Declaring the sysret pointer as uint64_t * is therefore misleading and
> forces callers to launder the value through an unsigned temporary.
>
> Change the sysret pointer to int64_t * across the public plugin API
> typedef (qemu_plugin_vcpu_syscall_filter_cb_t), the internal
> qemu_plugin_vcpu_syscall_filter() prototypes and stub, its
> implementation in plugins/core.c, the linux-user caller, and the
> in-tree example plugins.
>
> Signed-off-by: Ziyang Zhang <functioner@sjtu.edu.cn>

Acked-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro