Re: [PATCH] seccomp: Get actual errno value from failed seccomp functions

[Philippe Mathieu-Daudé]

Cc'ing Daniel (maintainer)

On 25/10/22 13:59, Michal Privoznik wrote:

Upon failure, a libseccomp API returns actual errno value very
rarely. Fortunately, after its commit 34bf78ab (contained in
2.5.0 release), the SCMP_FLTATR_API_SYSRAWRC attribute can be set
which makes subsequent APIs return true errno on failure.

This is especially critical when seccomp_load() fails, because
generic -ECANCELED says nothing.

Signed-off-by: Michal Privoznik 
---
  meson.build| 9 +
  softmmu/qemu-seccomp.c | 9 +
  2 files changed, 18 insertions(+)

diff --git a/meson.build b/meson.build
index b686dfef75..5f114c89d9 100644
--- a/meson.build
+++ b/meson.build
@@ -636,10 +636,16 @@ if vmnet.found() and not 
cc.has_header_symbol('vmnet/vmnet.h',
  endif
  
  seccomp = not_found

+seccomp_has_sysrawrc = false
  if not get_option('seccomp').auto() or have_system or have_tools
seccomp = dependency('libseccomp', version: '>=2.3.0',
 required: get_option('seccomp'),
 method: 'pkg-config', kwargs: static_kwargs)
+  if seccomp.found()
+seccomp_has_sysrawrc = cc.has_header_symbol('seccomp.h',
+'SCMP_FLTATR_API_SYSRAWRC',
+dependencies: seccomp)
+  endif
  endif
  
  libcap_ng = not_found

@@ -1849,6 +1855,9 @@ config_host_data.set('CONFIG_RDMA', rdma.found())
  config_host_data.set('CONFIG_SDL', sdl.found())
  config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
  config_host_data.set('CONFIG_SECCOMP', seccomp.found())
+if seccomp.found()
+  config_host_data.set('CONFIG_SECCOMP_SYSRAWRC', seccomp_has_sysrawrc)
+endif
  config_host_data.set('CONFIG_SNAPPY', snappy.found())
  config_host_data.set('CONFIG_TPM', have_tpm)
  config_host_data.set('CONFIG_USB_LIBUSB', libusb.found())
diff --git a/softmmu/qemu-seccomp.c b/softmmu/qemu-seccomp.c
index deaf8a4ef5..05abf257c4 100644
--- a/softmmu/qemu-seccomp.c
+++ b/softmmu/qemu-seccomp.c
@@ -312,6 +312,15 @@ static int seccomp_start(uint32_t seccomp_opts, Error 
**errp)
  goto seccomp_return;
  }
  
+#if defined(CONFIG_SECCOMP_SYSRAWRC)


Maybe add some comment here such:

 /*
  * This must be the first seccomp_attr_set() call to have full
  * error propagation.
  */


+rc = seccomp_attr_set(ctx, SCMP_FLTATR_API_SYSRAWRC, 1);
+if (rc != 0) {
+error_setg_errno(errp, -rc,
+ "failed to set seccomp rawrc attribute");
+goto seccomp_return;
+}
+#endif
+
  rc = seccomp_attr_set(ctx, SCMP_FLTATR_CTL_TSYNC, 1);
  if (rc != 0) {
  error_setg_errno(errp, -rc,


Reviewed-by: Philippe Mathieu-Daudé 

[PATCH] seccomp: Get actual errno value from failed seccomp functions

[Michal Privoznik]

Upon failure, a libseccomp API returns actual errno value very
rarely. Fortunately, after its commit 34bf78ab (contained in
2.5.0 release), the SCMP_FLTATR_API_SYSRAWRC attribute can be set
which makes subsequent APIs return true errno on failure.

This is especially critical when seccomp_load() fails, because
generic -ECANCELED says nothing.

Signed-off-by: Michal Privoznik 
---
 meson.build| 9 +
 softmmu/qemu-seccomp.c | 9 +
 2 files changed, 18 insertions(+)

diff --git a/meson.build b/meson.build
index b686dfef75..5f114c89d9 100644
--- a/meson.build
+++ b/meson.build
@@ -636,10 +636,16 @@ if vmnet.found() and not 
cc.has_header_symbol('vmnet/vmnet.h',
 endif
 
 seccomp = not_found
+seccomp_has_sysrawrc = false
 if not get_option('seccomp').auto() or have_system or have_tools
   seccomp = dependency('libseccomp', version: '>=2.3.0',
required: get_option('seccomp'),
method: 'pkg-config', kwargs: static_kwargs)
+  if seccomp.found()
+seccomp_has_sysrawrc = cc.has_header_symbol('seccomp.h',
+'SCMP_FLTATR_API_SYSRAWRC',
+dependencies: seccomp)
+  endif
 endif
 
 libcap_ng = not_found
@@ -1849,6 +1855,9 @@ config_host_data.set('CONFIG_RDMA', rdma.found())
 config_host_data.set('CONFIG_SDL', sdl.found())
 config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
 config_host_data.set('CONFIG_SECCOMP', seccomp.found())
+if seccomp.found()
+  config_host_data.set('CONFIG_SECCOMP_SYSRAWRC', seccomp_has_sysrawrc)
+endif
 config_host_data.set('CONFIG_SNAPPY', snappy.found())
 config_host_data.set('CONFIG_TPM', have_tpm)
 config_host_data.set('CONFIG_USB_LIBUSB', libusb.found())
diff --git a/softmmu/qemu-seccomp.c b/softmmu/qemu-seccomp.c
index deaf8a4ef5..05abf257c4 100644
--- a/softmmu/qemu-seccomp.c
+++ b/softmmu/qemu-seccomp.c
@@ -312,6 +312,15 @@ static int seccomp_start(uint32_t seccomp_opts, Error 
**errp)
 goto seccomp_return;
 }
 
+#if defined(CONFIG_SECCOMP_SYSRAWRC)
+rc = seccomp_attr_set(ctx, SCMP_FLTATR_API_SYSRAWRC, 1);
+if (rc != 0) {
+error_setg_errno(errp, -rc,
+ "failed to set seccomp rawrc attribute");
+goto seccomp_return;
+}
+#endif
+
 rc = seccomp_attr_set(ctx, SCMP_FLTATR_CTL_TSYNC, 1);
 if (rc != 0) {
 error_setg_errno(errp, -rc,
-- 
2.37.4