system: Forbid alloca()

[PATCH v2 1/3] target/ppc/kvm: Avoid using alloca()

Posted by Philippe Mathieu-Daudé 3 weeks, 6 days ago

kvmppc_load_htab_chunk() is used for migration, thus is not
a hot path. Use the heap instead of the stack, removing the
alloca() call.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 target/ppc/kvm.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index d145774b09a..937b9ee986d 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -2760,11 +2760,10 @@ int kvmppc_save_htab(QEMUFile *f, int fd, size_t bufsize, int64_t max_ns)
 int kvmppc_load_htab_chunk(QEMUFile *f, int fd, uint32_t index,
                            uint16_t n_valid, uint16_t n_invalid, Error **errp)
 {
-    struct kvm_get_htab_header *buf;
     size_t chunksize = sizeof(*buf) + n_valid * HASH_PTE_SIZE_64;
+    g_autofree struct kvm_get_htab_header *buf = g_malloc(chunksize);
     ssize_t rc;
 
-    buf = alloca(chunksize);
     buf->index = index;
     buf->n_valid = n_valid;
     buf->n_invalid = n_invalid;
-- 
2.51.0

Re: [PATCH v2 1/3] target/ppc/kvm: Avoid using alloca()

Posted by Stefan Hajnoczi 3 weeks, 5 days ago

On Mon, Sep 01, 2025 at 03:26:24PM +0200, Philippe Mathieu-Daudé wrote:
> kvmppc_load_htab_chunk() is used for migration, thus is not
> a hot path. Use the heap instead of the stack, removing the
> alloca() call.
> 
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  target/ppc/kvm.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Re: [PATCH v2 1/3] target/ppc/kvm: Avoid using alloca()

Posted by Harsh Prateek Bora 3 weeks, 5 days ago


On 9/1/25 18:56, Philippe Mathieu-Daudé wrote:
> kvmppc_load_htab_chunk() is used for migration, thus is not
> a hot path. Use the heap instead of the stack, removing the
> alloca() call.
> 
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>   target/ppc/kvm.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index d145774b09a..937b9ee986d 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2760,11 +2760,10 @@ int kvmppc_save_htab(QEMUFile *f, int fd, size_t bufsize, int64_t max_ns)
>   int kvmppc_load_htab_chunk(QEMUFile *f, int fd, uint32_t index,
>                              uint16_t n_valid, uint16_t n_invalid, Error **errp)
>   {
> -    struct kvm_get_htab_header *buf;
>       size_t chunksize = sizeof(*buf) + n_valid * HASH_PTE_SIZE_64;
> +    g_autofree struct kvm_get_htab_header *buf = g_malloc(chunksize);
>       ssize_t rc;
>   
> -    buf = alloca(chunksize);
>       buf->index = index;
>       buf->n_valid = n_valid;
>       buf->n_invalid = n_invalid;

Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>

Re: [PATCH v2 1/3] target/ppc/kvm: Avoid using alloca()

Posted by Manos Pitsidianakis 3 weeks, 6 days ago

On Mon, Sep 1, 2025 at 4:27 PM Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> kvmppc_load_htab_chunk() is used for migration, thus is not
> a hot path. Use the heap instead of the stack, removing the
> alloca() call.
>
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  target/ppc/kvm.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index d145774b09a..937b9ee986d 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2760,11 +2760,10 @@ int kvmppc_save_htab(QEMUFile *f, int fd, size_t bufsize, int64_t max_ns)
>  int kvmppc_load_htab_chunk(QEMUFile *f, int fd, uint32_t index,
>                             uint16_t n_valid, uint16_t n_invalid, Error **errp)
>  {
> -    struct kvm_get_htab_header *buf;
>      size_t chunksize = sizeof(*buf) + n_valid * HASH_PTE_SIZE_64;
> +    g_autofree struct kvm_get_htab_header *buf = g_malloc(chunksize);
>      ssize_t rc;
>
> -    buf = alloca(chunksize);
>      buf->index = index;
>      buf->n_valid = n_valid;
>      buf->n_invalid = n_invalid;
> --
> 2.51.0
>
>

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

Re: [PATCH v2 1/3] target/ppc/kvm: Avoid using alloca()

Posted by Peter Maydell 3 weeks, 6 days ago

On Mon, 1 Sept 2025 at 14:26, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> kvmppc_load_htab_chunk() is used for migration, thus is not
> a hot path. Use the heap instead of the stack, removing the
> alloca() call.
>
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  target/ppc/kvm.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index d145774b09a..937b9ee986d 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2760,11 +2760,10 @@ int kvmppc_save_htab(QEMUFile *f, int fd, size_t bufsize, int64_t max_ns)
>  int kvmppc_load_htab_chunk(QEMUFile *f, int fd, uint32_t index,
>                             uint16_t n_valid, uint16_t n_invalid, Error **errp)
>  {
> -    struct kvm_get_htab_header *buf;
>      size_t chunksize = sizeof(*buf) + n_valid * HASH_PTE_SIZE_64;
> +    g_autofree struct kvm_get_htab_header *buf = g_malloc(chunksize);
>      ssize_t rc;
>
> -    buf = alloca(chunksize);
>      buf->index = index;
>      buf->n_valid = n_valid;
>      buf->n_invalid = n_invalid;

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM