The use of a real syscall number causes problem with seccomp enabled
since we might not get a second chance in sysexit to set the return value from the syscall.
This, along with the use of PROOT_SET_SYSCALL,
was introduced in e675d09b2ce133110afe32ac379868d1691b1fdf.
The comment and the commit message claims that the syscall number must be a valid one.
However, based on local testing (on both an aarch64 kernel and an arm kernel)
it seems that while the 32bit kernel is unhappy about a invalid syscall number
it treats a -1 from the ptracer as ignoring the syscall just as what we need.
OTOH, the 64bit kernel doesn't seem to care at all and will happily treate the -2
as ignoring syscall as well. I have not found any document
about the special treatment of syscall(-1) but this is what strace uses.
The original choice of -2 seems to be because -1 is used to indicate uninitialized value.
It doesn't seem that this is needed anymore after 63c7f7f66824660408abc06b2b0ea98a00ef8096
and we should be able to use the better supported -1 for this purpose.
The use of a real syscall number causes problem with seccomp enabled since we might not get a second chance in sysexit to set the return value from the syscall.
This, along with the use of PROOT_SET_SYSCALL, was introduced in e675d09b2ce133110afe32ac379868d1691b1fdf. The comment and the commit message claims that the syscall number must be a valid one. However, based on local testing (on both an aarch64 kernel and an arm kernel) it seems that while the 32bit kernel is unhappy about a invalid syscall number it treats a
-1from the ptracer as ignoring the syscall just as what we need. OTOH, the 64bit kernel doesn't seem to care at all and will happily treate the-2as ignoring syscall as well. I have not found any document about the special treatment ofsyscall(-1)but this is what strace uses.The original choice of
-2seems to be because-1is used to indicate uninitialized value. It doesn't seem that this is needed anymore after 63c7f7f66824660408abc06b2b0ea98a00ef8096 and we should be able to use the better supported-1for this purpose.