[v3,1/1] libcamera: process: Pass stderr and reserve stdin and stdout fds
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Message ID 20250519122352.1675360-2-julien.vuillaumier@nxp.com
State New
Headers show
Series
  • libcamera: process: Pass stderr and reserve stdin and stdout fds
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Commit Message

Julien Vuillaumier May 19, 2025, 12:23 p.m. UTC 
When a child process is started from Process::start(), the file
descriptors inherited from the parent process are closed, except
the ones explicitly listed in the fds[] argument.

One issue is that the file descriptors for stdin, stdout and stderr
being closed, the subsequent file descriptors created by the child
process will reuse the values 0, 1 and 2 that are now available.
Thus, usage of printf(), assert() or alike may direct its output
to the new resource bound to one of these reused file descriptors.
The other issue is that the child process can no longer log on
the console because stderr has been closed.

To address the 2 issues, Process:start() is amended as below:
- Child process inherits from parent's stderr fd in order to share
the same logging descriptor
- Child process stdin, stdout and stderr fds are bound to /dev/null
if not inherited from parent. That is to prevent those descriptors
to be reused for any other resource, that could be corrupted by
the presence of printf(), assert() or alike.

Signed-off-by: Julien Vuillaumier <julien.vuillaumier@nxp.com>
---
 src/libcamera/process.cpp | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

Patch
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diff --git a/src/libcamera/process.cpp b/src/libcamera/process.cpp
index 68fad327..fa92f58c 100644
--- a/src/libcamera/process.cpp
+++ b/src/libcamera/process.cpp
@@ -259,7 +259,21 @@  int Process::start(const std::string &path,
 		if (isolate())
 			_exit(EXIT_FAILURE);
 
-		closeAllFdsExcept(fds);
+		std::vector<int> v(fds);
+		v.push_back(STDERR_FILENO);
+		closeAllFdsExcept(v);
+
+		const auto tryDevNullLowestFd = [](int expected, int oflag) {
+			int fd = open("/dev/null", oflag);
+			if (fd < 0)
+				exit(EXIT_FAILURE);
+			if (fd != expected)
+				close(fd);
+		};
+
+		tryDevNullLowestFd(STDIN_FILENO, O_RDONLY);
+		tryDevNullLowestFd(STDOUT_FILENO, O_WRONLY);
+		tryDevNullLowestFd(STDERR_FILENO, O_WRONLY);
 
 		const char *file = utils::secure_getenv("LIBCAMERA_LOG_FILE");
 		if (file && strcmp(file, "syslog"))