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authorH.J. Lu <hjl.tools@gmail.com>2021-02-01 11:00:38 -0800
committerH.J. Lu <hjl.tools@gmail.com>2021-02-01 11:00:52 -0800
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sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]
Add _SC_MINSIGSTKSZ for the minimum signal stack size derived from AT_MINSIGSTKSZ, which is the minimum number of bytes of free stack space required in order to gurantee successful, non-nested handling of a single signal whose handler is an empty function, and _SC_SIGSTKSZ which is the suggested minimum number of bytes of stack space required for a signal stack. If AT_MINSIGSTKSZ isn't available, sysconf (_SC_MINSIGSTKSZ) returns MINSIGSTKSZ. On Linux/x86 with XSAVE, the signal frame used by kernel is composed of the following areas and laid out as: ------------------------------ | alignment padding | ------------------------------ | xsave buffer | ------------------------------ | fsave header (32-bit only) | ------------------------------ | siginfo + ucontext | ------------------------------ Compute AT_MINSIGSTKSZ value as size of xsave buffer + size of fsave header (32-bit only) + size of siginfo and ucontext + alignment padding. If _SC_SIGSTKSZ_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ are redefined as /* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ). */ # undef SIGSTKSZ # define SIGSTKSZ sysconf (_SC_SIGSTKSZ) /* Minimum stack size for a signal handler: SIGSTKSZ. */ # undef MINSIGSTKSZ # define MINSIGSTKSZ SIGSTKSZ Compilation will fail if the source assumes constant MINSIGSTKSZ or SIGSTKSZ. The reason for not simply increasing the kernel's MINSIGSTKSZ #define (apart from the fact that it is rarely used, due to glibc's shadowing definitions) was that userspace binaries will have baked in the old value of the constant and may be making assumptions about it. For example, the type (char [MINSIGSTKSZ]) changes if this #define changes. This could be a problem if an newly built library tries to memcpy() or dump such an object defined by and old binary. Bounds-checking and the stack sizes passed to things like sigaltstack() and makecontext() could similarly go wrong.
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diff --git a/manual/conf.texi b/manual/conf.texi
index f959b00..ba9847a 100644
--- a/manual/conf.texi
+++ b/manual/conf.texi
@@ -913,6 +913,27 @@ Inquire about the parameter corresponding to @code{NL_SETMAX}.
@item _SC_NL_TEXTMAX
@standards{X/Open, unistd.h}
Inquire about the parameter corresponding to @code{NL_TEXTMAX}.
+
+@item _SC_MINSIGSTKSZ
+@standards{GNU, unistd.h}
+Inquire about the minimum number of bytes of free stack space required
+in order to guarantee successful, non-nested handling of a single signal
+whose handler is an empty function.
+
+@item _SC_SIGSTKSZ
+@standards{GNU, unistd.h}
+Inquire about the suggested minimum number of bytes of stack space
+required for a signal stack.
+
+This is not guaranteed to be enough for any specific purpose other than
+the invocation of a single, non-nested, empty handler, but nonetheless
+should be enough for basic scenarios involving simple signal handlers
+and very low levels of signal nesting (say, 2 or 3 levels at the very
+most).
+
+This value is provided for developer convenience and to ease migration
+from the legacy @code{SIGSTKSZ} constant. Programs requiring stronger
+guarantees should avoid using it if at all possible.
@end vtable
@node Examples of Sysconf